Uncouth old swimmer hogging second of lanes (6)
I believe the answer is:
oafish
'uncouth' is the definition.
(I've seen this before)
'old swimmer hogging second of lanes' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'swimmer' becomes 'fish' (fish swim).
'hogging' is an insertion indicator.
'second of lanes' becomes 'a' (2nd letter of 'lanes').
'o'+'fish'='ofish'
'ofish' going around 'a' is 'OAFISH'.
(Other definitions for oafish that I've seen before include "crass" , "Boorish, clumsy" , "Like a lout" , "Bone-headed" , "Uncouth" .)