Force in London may be encountered (3)
I believe the answer is:
met
This clue is a double definition.
'force in london' is the first definition.
(Metropolitan Police Service of London)
'be encountered' is the second definition.
(I've seen this before)
'may' acts as a link.
(Other definitions for met that I've seen before include "Encountered by chance or arrangement" , "Bumped into" , "satisfied" , "Encountered pro tem" , "Encountered or touched" .)