American nutmeg assortment - swell! (7)
I believe the answer is:
augment
'swell' is the definition.
(I've seen this in another clue)
'american nutmeg assortment' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'assortment' is an anagram indicator.
'nutmeg' anagrammed gives 'ugment'.
'a'+'ugment'='AUGMENT'
(Other definitions for augment that I've seen before include "Grow - intensify" , "Make larger" , "Expand" , "Add to, increase" , "Supplement" .)