Valve left out of timer (4)
I believe the answer is:
cock
'valve' is the definition.
(thesaurus)
'left out of timer' is the wordplay.
'left' becomes 'l' (common abbreviation).
'out of' suggests deleting specific letters ('out of' can mean no longer having).
'timer' becomes 'clock'.
'clock' with 'l' taken out is 'COCK'.
(Other definitions for cock that I've seen before include "Prepare to shoot" , "Tilt - tap" , "early bird" , "Pull back a gun's hammer in readiness to shoot" , "Small heap of hay" .)