Become friendly with a beginner in the police (4)
I believe the answer is:
melt
'become friendly' is the definition.
I can't judge whether this definition defines the answer.
'beginner in the police' is the wordplay.
'beginner' becomes 'L' (as in a learner driver with L-plates on their car).
'in the' is an insertion indicator.
'police' becomes 'met' (London Metropolitan Police).
'l' placed into 'met' is 'MELT'.
'with a' acts as a link.
Can you help me to learn more?
(Other definitions for melt that I've seen before include "Liquefy; become tender" , "Become liquefied" , "Calm" , "Dissolve with heat" , "Thaw" .)