A chap consumed with envy heading for a swimmer (7)
I believe the answer is:
manatee
'a swimmer' is the definition.
(I have seen 'Large swimmer' mean 'manatee' so perhaps 'swimmer' could also mean 'manatee')
'a chap consumed with envy heading' is the wordplay.
'a chap' becomes 'man' (chap can mean a man).
'consumed' becomes 'ate' (to eat is to consume food).
'with' means one lot of letters go next to another.
'heading' says to take the initial letters (head could mean the top letter).
The first letter of 'envy' is 'e'.
'man'+'ate'+'e'='MANATEE'
'for' acts as a link.
(Other definitions for manatee that I've seen before include "Sea denizen" , "Beast" , "rare sea mammal" , "A neat me (anag) - large aquatic mammal" , "water animal" .)