Most extreme bother on small driving area (8)
'most extreme' is the definition.
(at the most extreme angle)
'bother on small driving area' is the wordplay.
'bother' becomes 'pest'.
'on' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'driving area' becomes 'tee' (a golfer drives a golf ball from a tee).
'pest' after 'stee' is 'STEEPEST'.
(Other definitions for steepest that I've seen before include "most severe", "Rising most sharply", "Most expensive", "extremely dear", "With the sharpest incline".)