What's Scalar Operator in the plan?
When you're looking to track down the root cause of RESOURCE_SEMAPHORE waits, the plan operators you want to pay attention to are most commonly


Sorts
Hash Joins
Hash Match Aggregate


That's because it's a memory wait. Specifically, queries asking for memory, and SQL Server not having it available for them to start running. 

If you're on a more recent version of SQL Server (2012 SP3, 2014 SP2, 2016+), you can use a free tool like sp_BlitzCache (full disclosure, I'm a contributor) and run it to sort your plan cache by memory grant.

EXEC sp_BlitzCache @SortOrder = 'memory grant'


But fair warning, if you're hitting a bunch of that wait, there's a good chance your plan cache is wiped out from the memory pressure.