Constrict (7)
I believe the answer is:
tighten
'constrict' is the definition.
(I know that constrict is a more specific form of the action the tighten)
This is all the clue.
(Other definitions for tighten that I've seen before include "becoming tense" , "Make less loose" , "Constrict" , "Make (rope) less slack" , "Restrict" .)