It's tense but what's ahead couldn't be better (6,7)
I believe the answer is:
'it's tense' is the definition.
(future perfect is a kind of tense)
'what's ahead couldn't be better' is the wordplay.
'what's ahead' becomes 'future' (time that's ahead of the present).
'couldn't be better' becomes 'perfect'.
'but' acts as a link.
(Other definitions for future perfect that I've seen before include "of which I shall have studied an example" , "Utopian prospect" , "Grammatical verb tense" , "optimistic forecast?" , "Tense of verbs" .)