Excessive butt on one girl (8)

I believe the answer is:

prodigal

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'excessive' is the definition.
(thesaurus)

'butt on one girl' is the wordplay.
'butt on one' becomes 'prodi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'girl' becomes 'gal'.
'prodi'+'gal'='PRODIGAL'

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(Other definitions for prodigal that I've seen before include "overspending" , "Spendthrift" , "Recklessly wasteful or extravagant" , "Wasteful - gold pair (anag)" , "Extravagant like Biblical son" .)

I've seen this clue in The Sunday Times and The Times.

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