Bundle of fibres in cheek (5)
I believe the answer is:
nerve
I believe this clue is a double definition.
'bundle of fibres' is the first definition.
(I've seen this before)
'cheek' is the second definition.
(synonyms)
'in' is the link.
(Other definitions for nerve that I've seen before include "Brass - steel" , "Cool courage" , "Impudence; sensory fibre" , "Mettle, daring" , "Impudence; courage" .)