A ground nutmeg’s swell (7)
I believe the answer is:
augment
'swell' is the definition.
(I've seen this before)
'a ground nutmeg's' is the wordplay.
'ground' is an anagram indicator ('grind' the letters into a new form).
'nutmeg' is an anagram of 'ugment'.
'a'+'ugment'='AUGMENT'
(Other definitions for augment that I've seen before include "Make greater in size, number, etc." , "Grow - intensify" , "Add to, increase" , "Supplement" , "Enlarge or increase" .)