Come across London policemen arresting European (4)
I believe the answer is:
meet
'come across london' is the definition.
'meet' can be an answer for 'come' (meeting is a kind of coming). I'm unsure of the 'across london' bit.
'policemen arresting european' is the wordplay.
'policemen' becomes 'Met' (Metropolitan Police).
'arresting' indicates putting letters inside.
'european' becomes 'E' (abbreviation e.g. EU).
'met' going around 'e' is 'MEET'.
Can you help me to learn more?
(Other definitions for meet that I've seen before include "Fitting to come together" , "Happen upon" , "It is proper to encounter" , "Hunting gathering" , "Satisfy (requirements)" .)