A small sweet herb starting to go off in spread (8)
I believe the answer is:
pastille
'a small sweet' is the definition.
(I've seen this before)
'off in spread' is the wordplay.
'off' becomes 'ill'.
'in' means one lot of letters goes inside another.
'spread' becomes 'paste' ('paste' can be a synonym of 'spread'**).
'ill' going within 'paste' is 'PASTILLE'.
'herb starting to go' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for pastille that I've seen before include "Cough lozenge" , "Small sweet/lozenge" , "drop" , "List plea for the small sweet" , "Suck this" .)