Partly makes him merrily glisten (7)
I believe the answer is:
shimmer
'glisten' is the definition.
(I know this)
'partly makes him merrily' is the wordplay.
'partly' indicates the answer is hidden within the clue (part of the clue is used).
'SHIMMER' is hidden in the letters of 'makes him merrily'.
(Other definitions for shimmer that I've seen before include "Tinsel might do it" , "Give off tremulous light" , "Glisten (on water?)" , "Shine tremulously" , "glow" .)