Showing some cheek accepting easy extra work (12)

I believe the answer is:

moonlighting

'extra work' is the definition.

'showing some cheek accepting easy' is the wordplay.
'showing some cheek' becomes 'mooning' (to moon is to reveal your buttocks).
'accepting' means one lot of letters goes inside another.
'easy' becomes 'light' ('light' can be similar in meaning to 'easy').
'mooning' going around 'light' is 'MOONLIGHTING'.

(Another definition for moonlighting that I've seen is " Having a second job".)

I've seen this clue in The Guardian.

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