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 "Subject (of speech, etc)", "subject for discussion", "Content of argument", "Theme of discourse", "Something to discuss", "Question", "one discussed", "Theme (of, eg, a talk)", "Subject (of, eg, speech)" and "Subject (for conversation, etc)".)