Fibres used in thinner vest (6)
I believe the answer is:
nerves
'fibres' is the definition.
(I know this)
'in thinner vest' is the wordplay.
'in' says the answer is hidden in the clue.
'NERVES' can be found hidden inside 'thinner vest'.
'used' is the link.
(Other definitions for nerves that I've seen before include "They carry impulses between brain and body" , "Bodily fibres carrying sensations to brain" , "Feeling of anxiety" , "shows anxiety" , "Fibres that convey feelings" .)