Start breaking metal cross (5)
I believe the answer is:
tigon
'cross' is the definition.
(offspring of a lion and a tiger)
'start breaking metal' is the wordplay.
'start' becomes 'go' (synonyms).
'breaking' is an insertion indicator.
'metal' becomes 'tin' (tin is a kind of metal).
'go' placed within 'tin' is 'TIGON'.
(Other definitions for tigon that I've seen before include "Animal" , "Cross between two big cats" , "Cross between tiger and lion" , "Big hybrid of the cat family" , "cat cross" .)