Great tache! Trim sides and stick back on (8)

I believe the answer is:

reattach

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

'sides and stick back on' is the definition.
I can't judge whether this defines the answer.

'great tache trim' is the wordplay.
'trim' says the answer is hidden in the clue (I've seen 'trimmed' mean this (part of the clue must be trimmed away)).
'REATTACH' is hidden within 'great tache'.

Can you help me to learn more?

(Other definitions for reattach that I've seen before include "Once more assign" , "join up again" , "stick back" , "Fix back on" .)

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