easy way to get this is just to 1st make a save before you attack police patrol on the street
once that is done ambust them attack the police, just make sure to send a small party to make this quick and easy, once on the street they'll shoot your gangsters. dont shoot back, just keep skipping your turn once one of your gang is down to roughly 20-40% remaining HP a police office will run up to that gangster and cuff them. From there all left to do is ending the fight to your disadvantage (loosing or retreating) and the cuffed gangster is arrested = achievement.
there is a way to just retreat of the battle by going all the way the boarder of the battlescreen layout, you will find some 'darker' squares that seems alittle bitter darker than the usual movement square, have your remaining squad stands on those and once their turn is up, you can select "retreat"..the icon is like a speedy person running, should be right after or before the skipping your turn.
once the remaining squad leaves the battleground and that poor gangster was handcuffed, you will get it
update: i have made a video for this