Force top Frenchman to return in middle of site (6)
I believe the answer is:
Here is my best explanation:
'force' is the definition.
(impacting is a kind of forcing)
'top frenchman to return in middle of site' is the wordplay.
'top' becomes 'cap' (cap is a kind of top**).
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'to return' says the letters should be written backwards.
'in' means one lot of letters goes inside another.
'middle of' says to take the centre.
The middle letters of 'site' are 'it'.
'capm' reversed gives 'mpac'.
'mpac' placed inside 'it' is 'IMPACT'.
(Other definitions for impact that I've seen before include "Repercussion", "The force fo violent strike", "Collision - effect", "A big hit", "Forcible contact between two things", "Serious effect", "crash", "Crash", "Forceful consequence" and "Marked effect or influence".)