I must run, taking exercise as driving force (7)
I believe the answer is:
impetus
'driving force' is the definition.
(impetus is a kind of driving force)
'i must run taking exercise' is the wordplay.
'run' indicates anagramming the letters.
'taking' is an insertion indicator.
'exercise' becomes 'pe' (Physical Education).
'must' with letters rearranged gives 'mtus'.
'mtus' placed around 'pe' is 'mpetus'.
'i'+'mpetus'='IMPETUS'
'as' acts as a link.
(Other definitions for impetus that I've seen before include "Incentive - time's up (anag)" , "Force that moves something along" , "influence" , "Driving force, forward movement" , "Driving force, momentum" .)