Large stoppage hit officer first (7,6)
I believe the answer is:
general strike
'large stoppage' is the definition.
The definition suggests a singular noun which matches the answer.
'hit officer first' is the wordplay.
I cannot really understand how this works, but
'hit' could be 'strike' (synonyms) and 'strike' is found within the answer.
'officer' could be 'general' (general is a kind of officer) and 'general' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for general strike that I've seen before include "1926 withdrawal of labour" , "National work stoppage" , "Everybody out!" , "Everything stops for it" , "nationwide walkout" .)