Uncouth old swimmer crossing centre of bay (6)
I believe the answer is:
oafish
'uncouth' is the definition.
(I've seen this before)
'old swimmer crossing centre of bay' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'swimmer' becomes 'fish' (fish is a kind of aquatic vertebrate).
'crossing' means one lot of letters goes inside another.
'centre of' says to take the centre.
The central letter of 'bay' is 'a'.
'o'+'fish'='ofish'
'ofish' placed around 'a' is 'OAFISH'.
(Other definitions for oafish that I've seen before include "having poor manners" , "Boorish, clumsy" , "Bone-headed" , "Like a boorish person" , "Doltish" .)