Barnet's gone flat? You should 'ave this emergency action (7)
I believe the answer is:
airlift
'barnet's gone flat? you should 'ave this' is the definition.
The definition suggests a singular noun which matches the answer.
'emergency action' is the wordplay.
'emergency' becomes 'air' (I am not sure about this - if you are sure you should believe this answer much more).
'action' becomes 'lift' (lift is a kind of action).
'air'+'lift'='AIRLIFT'
Can you help me to learn more?
(Other definitions for airlift that I've seen before include "emergency procedure" , "Flying transportation" , "Transport by plane" , "means of escape" , "Rescue using planes" .)