Nerves? Lie and relax, briefly (6)
I believe the answer is:
fibres
'nerves?' is the definition.
(nerve is a kind of fibre)
'lie and relax briefly' is the wordplay.
'lie' becomes 'fib' (fib is a kind of lie).
'and' is a charade indicator (letters next to each other).
'relax' becomes 'rest' (to relax is to have a rest).
'briefly' means to remove the last letter.
'rest' with its last letter removed is 'res'.
'fib'+'res'='FIBRES'
(Other definitions for fibres that I've seen before include "Threadlike substances" , "thin strands or threads" , "Long thin substances, threads" , "Threadlike structures" , "Long slender substances" .)