Subject to choose, not the last (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'to choose not the last' is the wordplay.
'choose' becomes 'pick' (picking is a kind of choosing).
'not the last' means to remove the last letter.
'pick' with its last letter taken off is 'pic'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Subject (for conversation, etc)" , "Subject (eg, of text)" , "Matter for an argument" , "Subject matter of a conversation or discussion" , "Subject (of, eg, speech)" .)