Theme to select? Not entirely (5)
I believe the answer is:
topic
'theme' is the definition.
(I know that theme can be written as topic)
'to select? not entirely' is the wordplay.
'select?' becomes 'pick' (picking is a kind of selecting).
'not entirely' means to remove the last letter (not all of the word).
'pick' with its final letter removed is 'pic'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Subject matter of a conversation or discussion" , "Matter for an argument" , "Content of argument" , "Something to discuss" , "Issue (for discussion)" .)