For this reason free other crook (9)
I believe the answer is:
therefore
'for this reason' is the definition.
(I've seen this before)
'free other crook' is the wordplay.
'crook' indicates anagramming the letters (a crooked version of the letters).
'free'+'other'='freeother'
'freeother' with letters rearranged gives 'THEREFORE'.
(Other definitions for therefore that I've seen before include "as result" , "has a triangular formation in maths?" , "Ergo" , "It follows" , "In consequence" .)