Doctor can sue practice (6)
I believe the answer is:
usance
'practice' is the definition.
(I've seen this before)
'doctor can sue' is the wordplay.
'doctor' indicates an anagram.
'can'+'sue'='cansue'
'cansue' is an anagram of 'USANCE'.
(Other definitions for usance that I've seen before include "time to settle" , "period of reckoning" , "Time allowed to pay" , "old custom" , "interest in redemption" .)