Mother's interrupted by a bighead - the snake! (5)
I believe the answer is:
mamba
'snake' is the definition.
(mamba is a kind of snake)
'mother's interrupted by a bighead' is the wordplay.
'mother' becomes 'mama' (mama informally means mother).
'interrupted by' is an insertion indicator.
'a bighead' becomes 'b' (the head letter of 'big').
'mama' enclosing 'b' is 'MAMBA'.
'the' acts as a link.
(Other definitions for mamba that I've seen before include "Venomous snake; rhymes with dance" , "Death-dealing African" , "serpent" , "venomous crawler" , "Venomous African tree snake" .)