Fighting to swallow iron biscuit! (5)
I believe the answer is:
wafer
'biscuit' is the definition.
(wafer is a kind of biscuit)
'fighting to swallow iron' is the wordplay.
'fighting' becomes 'war'.
'to swallow' is an insertion indicator.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'war' placed around 'fe' is 'WAFER'.
(Other definitions for wafer that I've seen before include "Ice cream biscuit" , "thin stuff" , "Very thin slice" , "A very thin plain biscuit" , "Piece of bread" .)