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 subsidiary indication.
'top' becomes 'cap' (cap is a kind of top**).
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'to return' is a reversal indicator.
'in' is an insertion indicator.
'middle of' says to take the centre.
The middle letters of 'site' are 'it'.
'capm' backwards is 'mpac'.
'mpac' placed inside 'it' is 'impact'.
(Other definitions for impact that I've seen before include "Crash", "Significance", "Forcible contact; marked influence", "Forceful consequence", "The striking of one body against another", "The striking of one thing against another", "Forceful contact between two objects", "Collision - effect", "Effect - collision" and "The force fo violent strike".)