When about to bite on the nut (6)
I believe the answer is:
cashew
'the nut' is the definition.
(cashew is a kind of nut)
'when about to bite' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'about' means one lot of letters goes inside another.
'to bite' becomes 'chew' (chew is a kind of bite).
'as' inserted inside 'chew' is 'CASHEW'.
'on' acts as a link.
(Other definitions for cashew that I've seen before include "Kidney shaped nut" , "Common type of nut" , "Kind of nut" , "Edible nut" , "nut-tree" .)