Add to sort of neat gum (7)
I believe the answer is:
augment
'add to' is the definition.
(I've seen this in another clue)
'sort of neat gum' is the wordplay.
'sort of' indicates an anagram.
'neat'+'gum'='neatgum'
'neatgum' anagrammed gives 'AUGMENT'.
(Other definitions for augment that I've seen before include "Enlarge or increase" , "Swell" , "Increase number of" , "Make greater in size, number, etc." , "Expand" .)