Crossword Genius

Moon, or half moon, seen above gym (4)

Ross

I believe the answer is:

mope

'moon' is the definition.
(I've seen this before)

'half moon seen above gym' is the wordplay.
'half' indicates taking half.
'seen above' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'gym' becomes 'pe' (Physical Education).
'moon' halved is 'mo'.
'mo'+'pe'='MOPE'

'or' is the link.

(Other definitions for mope that I've seen before include "Languish" , "Misery guts" , "Brood over a poem" , "Pine - sulk" , "Go gloomily about" .)

I've seen this clue in The Times.
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