Drive MEP about in new suit (7)
I believe the answer is:
impetus
'drive' is the definition.
(impetus is a kind of drive)
'mep about in new suit' is the wordplay.
'about' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'new' indicates an anagram.
'mep' with letters rearranged gives 'mpe'.
'suit' is an anagram of 'itus'.
'mpe' inserted within 'itus' is 'IMPETUS'.
(Other definitions for impetus that I've seen before include "Motivation" , "Incentive - time's up (anag)" , "Catalyst" , "Drive" , "Imputes to force with which a body moves" .)