Decorating technique of crooks in GBS (9)
I believe the answer is:
scrimshaw
'decorating technique' is the definition.
I can't tell whether this defines the answer.
'of crooks in gbs' is the wordplay.
'of crooks' becomes 'crims' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'gbs' becomes 'shaw' (I've seen this in another clue).
'crims' inserted within 'shaw' is 'SCRIMSHAW'.
Can you help me to learn more?
(Other definitions for scrimshaw that I've seen before include "handiwork" , "Carved work" , "Carving on whalebone or walrus tusk" .)