Policeman finally brought in to break up quarrel (3-2)
I believe the answer is:
run-in
'quarrel' is the definition.
(I know that quarrel can be written as run-in)
'policeman finally brought in to break up' is the wordplay.
'finally' suggests the final letters.
'brought in to' indicates putting letters inside.
'break up' becomes 'ruin' (synonyms. I am not sure about the 'up' bit.).
The last letter of 'policeman' is 'n'.
'n' placed within 'ruin' is 'RUN-IN'.
Can you help me to learn more?
(Other definitions for run-in that I've seen before include "Approach" , "Matches late in season" , "Dispute" , "Home straight" , "Fight, disagreement" .)