Subject of work kept in check, mostly (5)
I believe the answer is:
topic
'subject' is the definition.
(as in a topic for discussion)
'work kept in check mostly' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'kept in' means one lot of letters goes inside another.
'check mostly' becomes 'tic' (I can't justify this - if you can you should give a lot more credence to this answer).
'op' put within 'tic' is 'TOPIC'.
'of' is the link.
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Theme (of, eg, a talk)" , "Talking point" , "might this be hot?" , "Subject (of a talk, etc)" , "Subject matter of a conversation or discussion" .)