Subject to go for mostly (5)
I believe the answer is:
topic
'subject' is the definition.
('topic' can be a synonym of 'subject')
'to go for mostly' is the wordplay.
'go for' becomes 'pick'.
'mostly' means to remove the last letter.
'pick' with its last letter removed is 'pic'.
'to'+'pic'='TOPIC'
(Other definitions for topic that I've seen before include "Talking point" , "'Theme, subject (5)'" , "Theme (of, eg, a talk)" , "Matter for an argument" , "might this be hot?" .)