Suffer less than a turn (7)
I believe the answer is:
undergo
'suffer' is the definition.
(undergoing is a kind of suffering)
'less than a turn' is the wordplay.
'less than' becomes 'under' (I've seen this before).
'a turn' becomes 'go' ('go' can be a synonym of 'turn').
'under'+'go'='UNDERGO'
(Other definitions for undergo that I've seen before include "Endure for guerdon" , "Suffer reverse" , "Stand" , "Withstand" , "Endure or be subjected to" .)