Become more tense (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
tighten
'become more tense' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tighten that I've seen before include "Stiffen" , "put the squeeze on" , "Fasten more firmly" , "Give an extra turn" , "get tense" .)