Old whaler's work: capturing felons in Wash, possibly (9)
I believe the answer is:
scrimshaw
'old whaler's work' is the definition.
(design carved into whale bone)
'capturing felons in wash possibly' is the wordplay.
'capturing felons in' becomes 'scrim' (I am not sure about this - if you are sure you should believe this answer much more).
'possibly' indicates an anagram.
'wash' with letters rearranged gives 'shaw'.
'scrim'+'shaw'='SCRIMSHAW'
(Other definitions for scrimshaw that I've seen before include "handiwork" , "Carved work" , "Carving on whalebone or walrus tusk" .)