Sphere of influence restricted by very large sweetener (8)
I believe the answer is:
sorbitol
'sweetener' is the definition.
(I've seen this before)
'sphere of influence restricted by very large' is the wordplay.
'sphere of influence' becomes 'orbit' (I've seen this in another clue).
'restricted by' is an insertion indicator (inserted letters are restricted or trapped).
'very' becomes 'so' (both can mean 'extremely').
'large' becomes 'L' (eg in clothes sizes).
'orbit' going into 'so' is 'sorbito'.
'sorbito'+'l'='SORBITOL'
(Other definitions for sorbitol that I've seen before include "Sugar substitute - lost Biro (anag)" , "Sweetener" .)