Sun-fish with no tail (3)
I believe the answer is:
sol
'sun' is the definition.
(Latin for 'sun')
'fish with no tail' is the wordplay.
'fish' becomes 'sole' (sole is a kind of fish).
'with no tail' means to remove the last letter.
'sole' with its last letter taken off is 'SOL'.
(Other definitions for sol that I've seen before include "Note, old french coin" , "Mexican lager" , "A Latin sun" , "nearby star" , "Note; Peruvian currency" .)