The effects for one politician on performances (7)
I believe the answer is:
impacts
'the effects' is the definition.
(impact is a kind of effect)
'one politician on performances' is the wordplay.
'one' becomes 'I' (Roman numeral).
'politician' becomes 'MP' (Member of Parliament).
'on' says to put letters next to each other.
'performances' becomes 'acts' (act is a kind of performance).
'i'+'mp'+'acts'='IMPACTS'
'for' acts as a link.
(Other definitions for impacts that I've seen before include "Bumps into, collides with" , "Collisions of jolts" , "It's camp (anag)" .)