Subject to go for mostly (5)
I believe the answer is:
Here is my best explanation:
'subject' is the definition.
('topic' can be a synonym of 'subject')
'to go for mostly' is the subsidiary indication.
'go for' becomes 'pick'.
'mostly' means to remove the last letter.
'pick' with its last letter taken off is 'pic'.
(Other definitions for topic that I've seen before include "Issue (for discussion)", "Talking point", "Subject (of, eg, speech)", "Something to discuss", "Subject (of a talk, etc)", "Subject (eg, of text)", "Subject (of speech, etc)", "Subject (for conversation, etc)", "A subject for conversation or debate" and "Theme of discourse".)