Be quiet about thickness returning in spirit (5)
I believe the answer is:
sylph
'spirit' is the definition.
(I've seen this before)
'be quiet about thickness returning' is the wordplay.
'be quiet' becomes 'sh'.
'about' is an insertion indicator.
'thickness' becomes 'ply' (I have seen 'Thickness of yarn ' mean 'ply' so perhaps 'thickness' could also mean 'ply').
'returning' shows that the letters should be reversed in order.
'ply' back-to-front is 'ylp'.
'sh' enclosing 'ylp' is 'SYLPH'.
'in' acts as a link.
(Other definitions for sylph that I've seen before include "Slim female" , "Thin and graceful female" , "Slender, graceful female" , "slender lass!" , "Imaginary spirit of the air" .)