Future perfect? Could be stressful! (5)
I believe the answer is:
tense
I believe this clue is a double definition.
'future perfect? could be' is the first definition.
(I've seen this before)
'stressful' is the second definition.
(tense can mean stressful)
(Other definitions for tense that I've seen before include "Rigid, or grammatical time" , "Stretched tightly, unable to relax" , "Strained by time in grammar" , "Present, possibly" , "(Of muscle) tight" .)