Men in Civil Defence backing current agreement (6)
I believe the answer is:
accord
'agreement' is the definition.
(accord is a kind of agreement)
'men in civil defence backing current' is the wordplay.
'men' becomes 'or' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'civil defence' becomes 'cd'.
'backing' is a charade indicator (letters next to each other).
'current' becomes 'ac' (alternating current).
'or' inserted inside 'cd' is 'cord'.
'cord' after 'ac' is 'ACCORD'.
(Other definitions for accord that I've seen before include "Model" , "Official agreement; harmony" , "binding agreement" , "Harmony, concurrence" , "Official treaty" .)