Part of London separates — to impact in some way (3,2)
I believe the answer is:
act on
'impact in some way' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how they can define each other.
'part of london separates' is the wordplay.
'part' becomes 'act' (I have seen 'Part of play ' mean 'act' so perhaps 'part' could also mean 'act').
'of' becomes 'o'.
'london separates' becomes 'n' (I can't justify this - if you can you should give a lot more credence to this answer).
'act'+'o'+'n'='ACT ON'
'to' is the link.
Can you help me to learn more?
(Other definitions for act on that I've seen before include "West London district" , "Take steps to follow - and affect" , "Carry out (instruction)" , "Part of the London borough of Ealing -- British historian, d. 1902" , "Comply with" .)