Only an excuse, but it 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 before). I'm unsure of the 'only an' bit.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for plea that I've seen before include "Defence - excuse - appeal" , "Earnest request, prayer" , "Request - appeal" , "Excuse, entreaty" , "response to criminal charges" .)

I've seen this clue in the Evening Standard.

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