Ready to work briefly at one time with everyone (2,4)
I believe the answer is:
on call
'ready to work' is the definition.
(someone on call is ready to work)
'briefly at one time with everyone' is the wordplay.
'briefly' means to remove the last letter.
'at one time' becomes 'once' (both can mean 'in the past').
'with' means one lot of letters go next to another.
'everyone' becomes 'all' (all people).
'once' with its final letter taken off is 'onc'.
'onc'+'all'='ON CALL'
(Other definitions for on call that I've seen before include "available in emergencies?" , "Available if needed" , "Contactable" , "ready to respond" , "Able to be contacted, like a GP" .)