Misery GI's left behind (5)

I believe the answer is:

grump

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'misery' is the definition.
(thesaurus)

'gi's left behind' is the wordplay.
'gi's left' becomes 'g' (I am not sure about this - if you are sure you should believe this answer much more).
'behind' becomes 'rump' (informal term for the buttocks).
'g'+'rump'='GRUMP'

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(Other definitions for grump that I've seen before include "Beef" , "Surly person" , "pet?" , "surly type" , "Sulky type" .)

I've seen this clue in The Daily Express.

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