Hostile to profits made in heartless act (7)
I believe the answer is:
against
'hostile' is the definition.
(thesaurus)
'profits made in heartless act' is the wordplay.
'profits' becomes 'gains' ('gain' can be a synonym of 'profit').
'made in' means one lot of letters goes inside another.
'heartless' says to hollow out the word (remove centre letters).
'act' with its middle taken out is 'at'.
'gains' inserted inside 'at' is 'AGAINST'.
'to' is the link.
(Other definitions for against that I've seen before include "Disapproving of" , "Abutting" , "Versus - in contact with" , "Opposed" , "In competition with" .)