Crook from north in sack (4)
I believe the answer is:
bend
'crook' is the definition.
(I know that crook can be written as bend)
'north in sack' is the wordplay.
'north' becomes 'N' (abbreviation).
'in' is an insertion indicator.
'sack' becomes 'bed' ('in the sack' means 'in bed').
'n' inserted into 'bed' is 'BEND'.
'from' is the link.
(Other definitions for bend that I've seen before include "Arc" , "Influence, bow" , "Stoop; (heraldic) stripe" , "Country; bird" , "Lean forward" .)