Being a lout may seem nothing to a swimmer (6)
I believe the answer is:
oafish
'being a lout may seem' is the definition.
I can't tell whether this defines the answer.
'nothing to a swimmer' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'to' says to put letters next to each other.
'swimmer' becomes 'fish' (fish swim).
'o'+'a'+'fish'='OAFISH'
Can you help me to learn more?
(Other definitions for oafish that I've seen before include "Bone-headed" , "Like a lout" , "crass" , "Boorish, clumsy" , "Like a boorish person" .)