'Drive' that makes one MP a key figure (5)
I believe the answer is:
impel
'drive' is the definition.
(I know that drive is a more specific form of hit)
'one mp a key figure' is the wordplay.
'one' becomes 'i' (Roman numeral).
'a key' becomes 'E' (musical key).
'figure' becomes 'l' (I've seen this in other clues).
'i'+'mp'+'e'+'l'='IMPEL'
'that makes' acts as a link.
(Other definitions for impel that I've seen before include "Urge or force to action" , "Force into motion" , "Force forwards" , "Urge or propel" , "Make" .)