Force most of the decay (6)
I believe the answer is:
thrust
'force' is the definition.
(thrusting is a kind of forcing)
'most of the decay' is the wordplay.
'most of' means to remove the last letter.
'decay' becomes 'rust' (rusting is a kind of decaying).
'the' with its last letter taken away is 'th'.
'th'+'rust'='THRUST'
(Other definitions for thrust that I've seen before include "Pierce" , "Push forcibly, say sword" , "Sudden lunge or attack" , "Push powerfully; stab" , "Lunge forward" .)