The sun fish is tailless (3)
I believe the answer is:
sol
'the sun' is the definition.
(Latin for 'sun')
'fish is tailless' is the wordplay.
'fish' becomes 'sole' (sole is a kind of fish).
'is tailless' means to remove the last letter (the tail of the word is removed).
'sole' with its last letter taken away is 'SOL'.
(Other definitions for sol that I've seen before include "Personification of the sun" , "Mexican lager" , "nearby star" , "Note, old french coin" , "Note; Peruvian currency" .)