Gay old eccentric eager to return Cockney's fiver? (4,6)
I believe the answer is:
lady godiva
'cockney's fiver?' is the definition.
(Cockney slang for a five pound note)
'gay old eccentric eager to return' is the wordplay.
'eccentric' indicates anagramming the letters.
'eager' becomes 'avid' (synonyms).
'to return' is a reversal indicator.
'avid' reversed gives 'diva'.
'gay'+'old'='gayold'
'gayold' anagrammed gives 'ladygo'.
'ladygo'+'diva'='LADY GODIVA'
(Other definitions for lady godiva that I've seen before include "Tom peeped at her" , "Naked rider through Coventry" , "Wife of Leofric, an early streaker!" , "Only Peeping Tom saw her ride!" , "She rode bareback (and bare front!)" .)