Earl seen by lake in swimming togs? Nonsense (12)
I believe the answer is:
blatherskite
'earl' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'by lake in swimming togs? nonsense' is the wordplay.
'by' is a charade indicator (letters next to each other).
'lake in swimming togs?' becomes 'skite' (I can't justify this - if you can you should give a lot more credence to this answer).
'nonsense' becomes 'blather' (blather is a kind of nonsense).
'skite' after 'blather' is 'BLATHERSKITE'.
'seen' is the link.
(Other definitions for blatherskite that I've seen before include "(Person talking) nonsense -- a Lettish berk (anag)" , "Chatterer" , "pants" , "fool" .)