Shrink must be against heartless commitment (8)
I believe the answer is:
contract
'shrink' is the definition.
(contracting is a kind of shrinking)
'against heartless commitment' is the wordplay.
'against' becomes 'contra' (I've seen this before**).
'heartless' says to hollow out the word (remove centre letters).
'commitment' with its centre removed is 'ct'.
'contra'+'ct'='CONTRACT'
'must be' is the link.
(Other definitions for contract that I've seen before include "Become smaller" , "bargain" , "Shrink from the business agreement" , "settlement" , "Covenant" .)