Drive bishop away from bumpiest ground (7)
I believe the answer is:
impetus
'drive' is the definition.
(impetus is a drive to do something)
'bishop away from bumpiest ground' is the wordplay.
'bishop' becomes 'B' (chess abbreviation).
'away from' indicates named letters should be taken away.
'ground' indicates an anagram ('grind' the letters into a new form).
'bumpiest' with 'b' taken away is 'umpiest'.
'umpiest' is an anagram of 'IMPETUS'.
(Other definitions for impetus that I've seen before include "Driving force, momentum" , "Thrust" , "Moving force" , "Momentum, stimulus" , "Incentive, stimulus" .)