Bang on about small breakfast item (5)
I believe the answer is:
toast
'breakfast item' is the definition.
(I've seen this in another clue)
'bang on about small' is the wordplay.
'bang on' becomes 'to a T' (both can mean precisely or exactly right).
'about' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'toat' going around 's' is 'TOAST'.
(Other definitions for toast that I've seen before include "Cook by radiant heat" , "Brown under a grill" , "Raise a glass to (someone)" , "cooked 13 [BREAD]" , "Tribute (plus a clinking of glasses?)" .)