He's sad, possibly, in sunglasses (6)
I believe the answer is:
shades
'sunglasses' is the definition.
(synonyms)
'he's sad possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'hes'+'sad'='hessad'
'hessad' is an anagram of 'SHADES'.
'in' is the link.
(Other definitions for shades that I've seen before include "Old ghosts or modern sunglasses" , "Hip sunglasses or gloomy ghosts" , "'Tints, tones (6)'" , "Sunglasses (informal)" , "disguise, sometimes" .)