Only an excuse, but this sure is fun! (4)

I believe the answer is:

plea

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'only an excuse' is the definition.
'plea' can be an answer for 'excuse' (I've seen this in another clue). I am not sure about the 'only an' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for plea that I've seen before include "Urgent entreaty" , "Defence - excuse - appeal" , "Dispute" , "Petition or supplication" , "Lightly char" .)

I've seen this clue in the Evening Standard.

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