What is this weird format (R06L/CLRD62) in METAR for runway being cleared?
The format follows a standard, although this one is different because of the  R at the front...

The format is the "Runway State Group":

First two digits are the Runway Number, 88 means all runways, 99 means a "repetition of the last message as no new information received".

The third digit is the runway deposits, it tells you what is on the runway:


  0 = Clear and dry 
  1 = Damp 
  2 = Wet or water patches 
  3 = Rime or frost covered (depth normally less than 1mm) 
  4 = Dry snow 
  5 = Wet snow 
  6 = Slush 
  7 = Ice 
  8 = compacted or rolled snow 
  9 = frozen ruts or ridges 
  / = type of deposit not reported (e.g. due to rwy clearance in progress)


The Fourth digit is the extent of the contamination:


  1 = 10% or less 
  2 = 11% to 25% 
  5 = 26% to 50% 
  9 = 51% to 100% 
  / = not reported (e.g. due to rwy clearance in progress)


The fifth and sixth digits are the depth of deposit:


  00 = less than 1mm  
  01 = 1mm etc  


to


  90 = 90mm 
  91 = not used 
  92 = 10cm 
  93 = 15cm 
  94 = 20cm 
  95 = 25cm 
  96 = 30cm 
  97 = 35cm 
  98 = 40cm or more 
  99 = Rwy(s) non-operational due to snow, slush, ice, large drifts or runway clearance, but depth not reported. 
  // = depth of deposit operationally not significant or measurable.


And the seventh and eighth digits are the braking action:


  28 = friction coefficient 0.28 
  38 = friction coefficient 0.38


or


  91 = Braking action: Poor  
  92 = Braking action: Medium/Poor 
  93 = Braking action: Medium 
  94 = Braking action: Medium/Good 
  95 = Braking action: Good 
  99 = Figures unreliable (e.g. if equipment has been used which does not measure satisfactorily in slush or snow.) 
  // = Braking action not reported (e.g. runway not operational; aerodrome closed; etc.)


So we can take your example: R06L/CLRD62

Means

R06L : Runway 06 Left
/ : Deposit not reported (clearance in progress)
CLRD: Contamination ceases to exist, so two groups omitted
62: Friction coefficient 0.62

Source: Skybrary.aero But I can't find one definitive source with all the possible reporting in a METAR, sadly.

It seems that this "Runway State Group" is mostly used in European countries, I can't find an example of it used in the U.S.. Here is one good reference which I think covers 99% of the METAR data for Europe.