Swimmer Ray snatching grouper's tail (5)
I believe the answer is:
bream
'swimmer' is the definition.
(bream is a kind of aquatic vertebrate)
'ray snatching grouper's tail' is the wordplay.
'ray' becomes 'beam' (synonyms).
'snatching' means one lot of letters goes inside another.
'tail' indicates one should take the final letters.
The last letter of 'grouper' is 'r'.
'beam' going around 'r' is 'BREAM'.
(Other definitions for bream that I've seen before include "Fish - British classical guitarist, b. 1933" , "Amber (anag) - fish" , "Fish found in amber" , "Fish - English guitarist" , "Fish of the carp family" .)