Feline loses heart in row (4)
I believe the answer is:
tier
'row' is the definition.
(a tier is a row of things)
'feline loses heart' is the wordplay.
'feline' becomes 'tiger' (tiger is a kind of feline).
'loses heart' means to remove the middle letters.
'tiger' with its centre taken out is 'TIER'.
'in' acts as a link.
(Other definitions for tier that I've seen before include "Level of a structure" , "Bank - row" , "Rank - row" , "he hasn't won?" , "Wedding cake feature" .)