Nervous but could be perfect (5)
I believe the answer is:
tense
'nervous' is the definition.
(similar in meaning)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for tense that I've seen before include "Future, possibly" , "contract" , "Nerve-racking" , "Future perfect? Could be" , "Stressed" .)