Lapp gaining weight is a wise guy (5)
I believe the answer is:
swami
'a wise guy' is the definition.
(I've seen this in another clue)
'lapp gaining weight' is the wordplay.
'lapp' becomes 'sami' (synonyms).
'gaining' indicates putting letters inside.
'weight' becomes 'w' (short for weight).
'sami' enclosing 'w' is 'SWAMI'.
'is' acts as a link.
(Other definitions for swami that I've seen before include "Hindu leader" , "Indian religious teacher" , "Guru" , "Hindu male religious teacher" , "Hindu religious instructor" .)