Definitely confirm those at heart of fine family (3,2)
I believe the answer is:
ink in
'definitely confirm' is the definition.
(I've seen this before)
'heart of fine family' is the wordplay.
'heart of' indicates the central letters.
'family' becomes 'kin' (synonyms).
The centre of 'fine' is 'in'.
'in'+'kin'='INK IN'
'those at' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ink in that I've seen before include "Definitely confirm" , "Fill with colour" , "confirm initial answer, perhaps" , "Confirm arrangement" .)