Where prompt action may be needed behind the scenes (3-5)
I believe the answer is:
off-stage
'behind the scenes' is the definition.
(thesaurus)
'where prompt action may be' is the wordplay.
I cannot quite understand how this works, but
'action' could be 'stage' (stage is a kind of action) and 'stage' is present in the answer.
The remaining letters 'off' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'needed' acts as a link.
Can you help me to learn more?
(Another definition for off-stage that I've seen is " In the wings".)