Love excellent final letter (5)
I believe the answer is:
omega
'final letter' is the definition.
(I've seen this before)
'love excellent' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'excellent' becomes 'mega' (old-fashioned UK slang for 'very good').
'o'+'mega'='OMEGA'
(Other definitions for omega that I've seen before include "Athenian character" , "letter to Greeks" , "Last thing" , "Last letter of the Greek alphabet" , "Final Greek letter" .)