Gleam covers him: mercifully only partly (7)
I believe the answer is:
shimmer
'gleam' is the definition.
(I know that shimmer is a type of gleam)
'covers him mercifully only partly' is the wordplay.
'only partly' indicates a hidden word.
'SHIMMER' is hidden within 'covers him mercifully'.
(Other definitions for shimmer that I've seen before include "glow" , "Shine tremulously" , "Twinkle" , "glint"" , "Give off tremulous light" .)