Rail about dissolute fellow's sentimentality at first (9)
I believe the answer is:
corncrake
'rail' is the definition.
(corncrake is a kind of rail)
'about dissolute fellow's sentimentality at first' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'dissolute fellow' becomes 'rake' (historical term for an immoral man).
'sentimentality' becomes 'corn' (corniness).
'at first' is a charade indicator (letters next to each other).
'c'+'rake'='crake'
'crake' put after 'corn' is 'CORNCRAKE'.
(Other definitions for corncrake that I've seen before include "Bird" , "kind of rail" .)