Collision involving current member, one visiting part of W London (9)

'collision' is the definition.
(impaction is a kind of collision)

'current member one visiting part of w london' is the wordplay.
'current' becomes 'i' (symbol for electric current).
'member' becomes 'mp'.
'one' becomes 'i' (Roman numeral).
'visiting' means one lot of letters goes inside another.
'part of w london' becomes 'acton' (part of West London).
'i' put into 'acton' is 'action'.
'i'+'mp'+'action'='IMPACTION'

'involving' acts as a link.