Subject to endless choice (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'to endless choice' is the wordplay.
'endless' means to remove the last letter.
'choice' becomes 'pick' ('pick' can be a synonym of 'choice').
'pick' with its last letter taken away is 'pic'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Issue (for discussion)" , "Subject matter, theme" , "A subject for conversation or debate" , "That may be debatable" , "Something to discuss" .)