Fight to get iron into biscuit (5)
I believe the answer is:
wafer
'biscuit' is the definition.
(I know that wafer is a type of biscuit)
'fight to get iron' is the wordplay.
'fight' becomes 'war' (warring is a kind of fighting).
'to get' is an insertion indicator.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'war' enclosing 'fe' is 'WAFER'.
'into' is the link.
(Other definitions for wafer that I've seen before include "Light thin biscuit" , "A very thin plain biscuit" , "Thin, crisp biscuit or bread" , "Communion bread" , "Very thin 14 Down" .)