He doubts retracted chest muscles twitch (7)
I believe the answer is:
sceptic
'he doubts' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'retracted chest muscles twitch' is the wordplay.
'retracted' says the letters should be written in reverse.
'chest muscles' becomes 'pecs' (pectorals).
'twitch' becomes 'tic' (tic is a kind of twitch).
'pecs' back-to-front is 'scep'.
'scep'+'tic'='SCEPTIC'
Can you help me to learn more?
(Other definitions for sceptic that I've seen before include "Person who doubts the truth" , "Someone inclined to disbelieve" , "Agnostic" , "One is inclined to doubt accepted opinions" , "One might doubt it" .)