Ultimately devious, little chap a local pain? (6)
I believe the answer is:
stitch
'a local pain?' is the definition.
(pain local to the side of one's body)
'ultimately devious little chap' is the wordplay.
'ultimately' suggests the final letters.
'little chap' becomes 'titch' (term for a short person).
The last letter of 'devious' is 's'.
's'+'titch'='STITCH'
(Other definitions for stitch that I've seen before include "Link, loop or knot made in knitting or sewing" , "One in time can save nine, they say" , "Loop of thread" , "Surgical suture" , "Use a needle - pain in the side" .)