Additionally unpleasant smell's penetrating honk (2,4)
I believe the answer is:
to boot
'additionally' is the definition.
('to boot' can be a synonym of 'additionally')
'unpleasant smell's penetrating honk' is the wordplay.
'unpleasant smell' becomes 'bo' (informal abbreviation for body odour).
'penetrating' is an insertion indicator.
'honk' becomes 'toot' ('toot' can be a synonym of 'honk').
'bo' going into 'toot' is 'TO BOOT'.
(Other definitions for to boot that I've seen before include "Kick" , "Also, in addition - footwear?" , "furthermore" , "on top of everything else" , "In addition - using heavy footwear" .)