Extend neat mug design (7)
I believe the answer is:
augment
'extend' is the definition.
(thesaurus)
'neat mug design' is the wordplay.
'design' indicates an anagram.
'neat'+'mug'='neatmug'
'neatmug' with letters rearranged gives 'AUGMENT'.
(Other definitions for augment that I've seen before include "Make greater in size, number, etc." , "get longer" , "Expand" , "Increase number of" , "boost" .)