Best possible prediction, but it makes us tense! (6,7)
I believe the answer is:
future perfect
'tense' is the definition.
(future perfect tense is a kind of tense)
'best possible prediction but it' is the wordplay.
I cannot quite see how this works, but
'best' could be 'perfect' (associated in meaning) and 'perfect' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'makes us' is the link.
Can you help me to learn more?
(Other definitions for future perfect that I've seen before include "terse utopian forecast" , "with idealistic expectation?" , "Tense of verbs" , "Utopian prospect" , "of which I shall have studied an example" .)