Heartless cat in row (4)
I believe the answer is:
tier
'row' is the definition.
(a tier is a row of things)
'heartless cat' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'cat' becomes 'tiger' (tiger is a kind of cat).
'tiger' with its centre taken out is 'TIER'.
'in' is the link.
(Other definitions for tier that I've seen before include "Rank - order" , "band" , "Bank - row" , "One of two or more rows" , "Rank - row" .)