Clever swimmer home after hard trudge back (7)
I believe the answer is:
dolphin
'clever swimmer' is the definition.
(dolphins are highly intelligent)
'home after hard trudge back' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'after' means one lot of letters go next to another.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'trudge' becomes 'plod' (synonyms).
'back' shows that the letters should be reversed in order.
'h'+'plod'='hplod'
'hplod' reversed gives 'dolph'.
'in' put after 'dolph' is 'DOLPHIN'.
(Other definitions for dolphin that I've seen before include "Intelligent marine mammal" , "Sociable sea mammal, sacred in ancient Greece" , "Sea-mammal" , "Small toothed whale, larger than a porpoise" , "Sea mammal like large porpoise, friendly towards people" .)