One agreement following mass strike (6)
I believe the answer is:
impact
'strike' is the definition.
(I've seen this before)
'one agreement following mass' is the wordplay.
'one' becomes 'i' (Roman numeral).
'agreement' becomes 'pact' (pact is a kind of agreement).
'following' says to put letters next to each other.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'pact' put after 'm' is 'mpact'.
'i'+'mpact'='IMPACT'
(Other definitions for impact that I've seen before include "Force exerted by collision" , "The striking of one body against another" , "Strong effect" , "Forceful striking" , "Force of colllision or blow" .)