Back muscles twitch as lieutenant starts questioning (9)
I believe the answer is:
sceptical
'questioning' is the definition.
(I know that questioning can be written as sceptical)
'back muscles twitch as lieutenant starts' is the wordplay.
'back' is a reversal indicator.
'muscles' becomes 'pecs' (short for pectoral muscles).
'twitch' becomes 'tic' (tic is a kind of twitch).
'starts' says to take the initial letters.
The first letters of 'as lieutenant' is 'al'.
'pecs' backwards is 'scep'.
'scep'+'tic'+'al'='SCEPTICAL'
(Other definitions for sceptical that I've seen before include "Doubtful" , "Inclined to disbelieve, not convinced" , "dubious" , "Doubting or disbelieving" , "Inclined not to believe" .)