British PM once interrupted by a cross martinet (6-3)
I believe the answer is:
battle-axe
'martinet' is the definition.
'british pm once interrupted by a cross' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'pm once' becomes 'attlee' (Clement Attlee).
'interrupted by' indicates putting letters inside.
'cross' becomes 'x'.
'a'+'x'='ax'
'attlee' going around 'ax' is 'attleaxe'.
'b'+'attleaxe'='BATTLE-AXE'
(Other definitions for battle-axe that I've seen before include "Overbearing woman" , "Broad-bladed weapon - domineering woman" , "Old chopper- nagging old woman" , "Ancient combat weapon or very tough woman" , "A formidable and aggressive woman - with ancient weapon?" .)