Momentum member occupying fancy suite (7)
I believe the answer is:
impetus
'momentum' is the definition.
(I've seen this before)
'member occupying fancy suite' is the wordplay.
'member' becomes 'mp'.
'occupying' indicates putting letters inside.
'fancy' indicates anagramming the letters.
'suite' with letters rearranged gives 'ietus'.
'mp' placed within 'ietus' is 'IMPETUS'.
(Other definitions for impetus that I've seen before include "Catalyst" , "Force that moves something along" , "Incentive - time's up (anag)" , "Momentum, stimulus" , "Drive" .)