Accomplished over mile in leisurely walk (5)
I believe the answer is:
'leisurely walk' is the definition.
(I've seen this before)
'accomplished over mile' is the wordplay.
'accomplished' becomes 'able' (I've seen this before).
'over' means one lot of letters goes inside another.
'mile' becomes 'm' (common abbreviation).
'able' placed around 'm' is 'AMBLE'.
'in' acts as a link.
(Other definitions for amble that I've seen before include "Wander gently along" , "promenade" , "Relaxed walk" , "Walk gently" , "Toddler does this" .)