Showing strain (5)
I believe the answer is:
tense
'showing strain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tense that I've seen before include "Anxious - future, perhaps?" , "Future, possibly" , "Verb form, indicates time" , "It may be perfect" , "Strained - form of verb" .)