Source chum used occasionally, before heartless mob made them give in (7)
I believe the answer is:
succumb
'give in' is the definition.
(synonyms)
'source chum used occasionally before heartless mob' is the wordplay.
'source' indicates taking the first letters (I've seen 'sources of' mean this (the beginning of a word is its sauce)).
'occasionally' becomes 'suc' (I can't justify this - if you can you should give a lot more credence to this answer).
'before' says to put letters next to each other.
'heartless' suggests removing the centre.
'mob' with its centre taken out is 'mb'.
The first letters of 'chum used' is 'cu'.
'cu' after 'suc' is 'succu'.
'succu'+'mb'='SUCCUMB'
'made them' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for succumb that I've seen before include "Give way or sink under pressure" , "Be fatally overwhelmed" , "Fail to resist (temptation, perhaps)" , "Give in (to temptation etc)" , "Yield to pressure, temptation etc" .)