Lets out cry: 'Don't move a muscle!' (5)
I believe the answer is:
frees
'lets out' is the definition.
('free' can be a synonym of 'let out')
'cry don't move a muscle' is the wordplay.
'cry' indicates a 'sounds like' (homophone) clue (cried aloud).
'don't move a muscle' becomes 'freeze' ('freeze' is a command to stay still).
'freeze' is a homophone of 'FREES'.
(Other definitions for frees that I've seen before include "Liberates" , "Releases from confinement" , "Delivers" .)