Mannerly, fishy chap? (7)
I believe the answer is:
genteel
'mannerly' is the definition.
(I've seen this before)
'fishy chap?' is the wordplay.
I cannot quite see how this works, but
'chap?' could be 'gent' (I've seen this in other clues) and 'gent' is found within the answer.
The remaining letters 'eel' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
(Other definitions for genteel that I've seen before include ""Refayned"" , "Terribly polite" , "Affectedly refined or respectable" , "Exaggeratedly polite" , "Fashionable" .)