Surprise when mum bites into crackers (8)
I believe the answer is:
astonish
'surprise' is the definition.
(astonishing is a kind of surprising)
'when mum bites into crackers' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'mum' becomes 'sh' (both can mean 'keep quiet').
'bites' indicates putting letters inside.
'crackers' indicates anagramming the letters (informal term for mad).
'as'+'sh'='assh'
'into' anagrammed gives 'toni'.
'assh' placed around 'toni' is 'ASTONISH'.
(Other definitions for astonish that I've seen before include "Amaze, astound" , "Stun" , "Flabbergast" , "Take by surprise" , "Cause amazement" .)