Runs happen to be hard to come by (4)
I believe the answer is:
rare
'hard to come by' is the definition.
(I've seen this in another clue)
'runs happen to be' is the wordplay.
I cannot really understand how this works, but
'runs' could be 'r' (cricket abbreviation) and 'r' is found within the answer.
'be' could be 'are' (I've seen this in another clue) and 'are' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rare that I've seen before include "of low density" , "Special" , "like Ben Jonson, Jack Young thought" , "Very scarce, hardly cooked" , "Very scarce like valuable antique" .)