Wingback behind grouch (5)

I believe the answer is:

grump

I'm a little stuck... Click here to teach me more about this clue!

'grouch' is the definition.
(I know that grouch can be written as grump)

'wingback behind' is the wordplay.
'wingback' becomes 'g' (I can't justify this - if you can you should believe this answer much more).
'behind' becomes 'rump' (informal term for the buttocks).
'g'+'rump'='GRUMP'

Can you help me to learn more?

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

I've seen this clue in The Daily Express.

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