The most common and most likely answer to this crossword clue is the 9 letter word TENSIONED
I've seen this crossword clue in The Daily Telegraph
T
E
N
S
I
O
N
E
D
More clues leading to the result TENSIONED
Buddy explains!

The straight part
We know that "Under strain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case under strain = tensioned.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does TENSIONED mean?
TENSIONED verb- To place an object in tension, to pull or place strain on.
- In tension; strained or pulled on.