Tightens up (5)
I believe the answer is:
tones
'tightens up' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tones that I've seen before include "Told about the silly ass" , "Tints of colour" , "Qualities of people's voices" , "Notes, characters" , "Qualities of colour, music or speech" .)