Did long study, returning after some time (7)
I believe the answer is:
yearned
'did long' is the definition.
('yearn' can be a synonym of 'long')
'study returning after some time' is the wordplay.
'study' becomes 'den' (I've seen this before).
'returning' says the letters should be written backwards.
'after' is a charade indicator (letters next to each other).
'some time' becomes 'year' (I've seen this before**).
'den' reversed gives 'ned'.
'ned' after 'year' is 'YEARNED'.
(Other definitions for yearned that I've seen before include "Did long" , "Deanery (anag)" , "Pined" , "Ached" , "Longed (for something)" .)