Walking free of pain outside (8)
I believe the answer is:
striding
'walking' is the definition.
(striding is a kind of walking)
'free of pain outside' is the wordplay.
'free of' becomes 'rid' (I've seen this in another clue).
'pain' becomes 'sting' (sting is a kind of pain).
'outside' is an insertion indicator (some letters go outside others).
'rid' going inside 'sting' is 'STRIDING'.
(Other definitions for striding that I've seen before include "Stepping out" , "Walking with long steps" , "edge of the Lake District" .)