Fish love jargon (5)
I believe the answer is:
lingo
'jargon' is the definition.
(I know that jargon can be written as lingo)
'fish love' is the wordplay.
'fish' becomes 'ling' (ling is a kind of fish).
'love' becomes 'o' (love means zero in tennis).
'ling'+'o'='LINGO'
(Other definitions for lingo that I've seen before include "Language (slang)" , "French?" , "Patois" , "Jargon, foreign language (informal)" , "Language, colloquially speaking." .)