Groom pleased, slipped away (7)
I believe the answer is:
elapsed
'slipped away' is the definition.
('elapse' can be a synonym of 'slip away')
'groom pleased' is the wordplay.
'groom' indicates anagramming the letters.
'pleased' anagrammed gives 'ELAPSED'.
(Other definitions for elapsed that I've seen before include "Went by" , "Pleased (anag.)" , "Expired" , "Passed, of time" , "Pleased to have passed" .)