How to understand "实" in this context? ”五侯九伯，女实征之“（《齐桓公伐楚盟屈完》）之"实"字何以训？
Here's an expansion from 《史記》·《齊太公世家》that clarifies: 

「『東至海，西至河，南至穆陵，北至無棣，五侯九伯，實得征之。』齊由此得征伐，為大國。」 

In this expansion, 實 means "truly"/"legitimately" (實) authorized to (得) launch expeditions (征) against them (之): launch expeditions against them with actual authority.  So "legitimately" might be your best single word for the apparent function of 實 here.

實 as 的確 makes some sense in the expanded version, but still doesn't make sense as 的確 in the abbreviated form you cite in the 春秋左傳; so I would take that 實 as an abbreviated form of 實得, and gloss the original shorter phrase as "legitimately [authorized], go on the expedition" .  It would be interesting to interpret 實 as "具備"(權力) but I can't find any clear evidence for that reading.  

More examples of the 實得 usage would be useful: I'm having trouble finding other instances of this combination.  It's worth noting that the above interpretation works quite well for the only other similar usage of 實 I can find, also from 《春秋左傳》·《昭公三十年》:「靈王之喪，我先君簡公在楚，我先大夫印段實往，...」.

Other Examples

Here are some additional examples of uses we can use to examine the problem.  Based on searches from ctext.org.

《覲禮》:


  天子設斧依於戶牖之間，左右幾。天子袞冕，負斧依。嗇夫承命，告於天子。天子曰：「非他，伯父實來，予一人嘉之。伯父其入，予一人將受之。」侯氏入門右，坐奠圭，再拜稽首。擯者謁。侯氏坐取圭，升致命。


This seems useful because the gloss as 確實 seems even more strained than the example you have provided.  Here it seems to mean "May the Royal Protector [伯父] come here under authorization, so that We may congratulate him..." and then the Royal Protector comes in (I think - not sure what 侯氏 is, maybe one clan/surname/氏 of the 諸侯?).  

《周語中》


  王召士季，曰：「子弗聞乎，禘郊之事，則有全烝；王公立飫，則有房烝；親戚宴饗，則有肴烝。今女非他也，而叔父使士季實來修舊德，以獎王室。唯是先王之宴禮，欲以貽女。...


I.e., the Royal Uncle [叔父, again, whatever] has dispatched 士季 to come under orders to renew old good will, in order to favor the Royal House.  

There are other examples of "實來" in canon where it's not so clear it isn't simply meaning "really came". The advantage to the above selections, particularly the second, is that it's a real stretch to translate it as something like "X dispatched Y to actually visit..." - it just doesn't make very much sense.