Hidden downside (5)
I believe the answer is:
catch
'hidden downside' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for catch that I've seen before include "Dismiss (batsman); break (in voice)" , "Getting round" , "Arrest" , "a sort of 22" , "Stop the motion, of a ball say" .)