Sprain on a walk? (4)
I believe the answer is:
turn
I believe this is a double definition.
'sprain' is the first definition.
('turn' can be a synonym of 'sprain')
'a walk?' is the second definition.
(turn is a kind of walk)
'on' acts as a link.
(Other definitions for turn that I've seen before include "roll" , "Spell" , "Shift - wheel - round" , "See 19" , "Revolve; act" .)