Fish, ray, rounding rear of schooner (5)
I believe the answer is:
bream
'fish' is the definition.
(I know that bream is a type of fish)
'ray rounding rear of schooner' is the wordplay.
'ray' becomes 'beam' (synonyms).
'rounding' is an insertion indicator.
'rear of' suggests the final letters.
The last letter of 'schooner' is 'r'.
'beam' enclosing 'r' is 'BREAM'.
(Other definitions for bream that I've seen before include "Fish - English guitarist" , "Fish found in amber" , "Swimmer" , "River fish" , "Fish - British classical guitarist, b. 1933" .)