Force favourite stuffing into mouth, using spoon at first (7)
I believe the answer is:
impetus
'force' is the definition.
(impetus is a kind of force)
'favourite stuffing into mouth using spoon at first' is the wordplay.
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite'**).
'stuffing' indicates putting letters inside.
'at first' says to take the initial letters.
The initial letters of 'into mouth using spoon' is 'imus'.
'pet' placed within 'imus' is 'IMPETUS'.
(Other definitions for impetus that I've seen before include "Catalyst" , "Force that moves something along" , "Driving force, forward movement" , "Thrust" , "Momentum, stimulus" .)