Introduce bit of sugar into still (6)
I believe the answer is:
insert
'introduce bit' is the definition.
'insert' can be an answer for 'introduce' ('insert' can be a synonym of 'introduce'). I'm unsure of the 'bit' bit.
'sugar into still' is the wordplay.
'sugar' becomes 's' (this might be a standard abbreviation I've not previously seen).
'into' indicates putting letters inside.
'still' becomes 'inert' (I've seen this before).
's' placed into 'inert' is 'INSERT'.
'of' is the link.
(Other definitions for insert that I've seen before include "place in the interior" , "Enter" , "Enclose" , "Place in between other things" , "Put between or within other things" .)