Union leader overwhelmed by British politician’s impact (4)
I believe the answer is:
bump
'impact' is the definition.
(I know that bump is a type of impact)
'union leader overwhelmed by british politician's' is the wordplay.
'leader' says to take the initial letters.
'overwhelmed by' is an insertion indicator (inserted letters get absorbed or overwhelmed).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'politician' becomes 'MP' (Member of Parliament).
The first letter of 'union' is 'u'.
'b'+'mp'='bmp'
'u' going inside 'bmp' is 'BUMP'.
(Other definitions for bump that I've seen before include "Speed restriction" , "little impact" , "Jolt - collision - swelling" , "Bounce" , "Knock - collide" .)