Joined the police (3)
I believe the answer is:
met
This clue is a double definition.
'joined' is the first definition.
(meeting is a kind of joining)
'police' is the second definition.
(London Metropolitan Police)
'the' is the link.
(Other definitions for met that I've seen before include "Encountered by chance or arrangement" , "Encountered, came across" , "Came together, encountered" , "Got together" , "Encountered, came into contact" .)