Impact fellow with iron, in turn (6)
I believe the answer is:
affect
'impact' is the definition.
('affect' can be a synonym of 'impact')
'fellow with iron in turn' is the wordplay.
'fellow with' becomes 'f' (I've seen this in other clues).
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'in' means one lot of letters goes inside another.
'turn' becomes 'act' (theatre).
'f'+'fe'='ffe'
'ffe' inserted into 'act' is 'AFFECT'.
(Other definitions for affect that I've seen before include "Pretend to feel or arouse sadness" , "Touch (feelings); pretend (to feel)" , "Arouse sadness or sympathy like tear-jerker" , "Make impression on or arouse sympathy in" , "attack" .)