Sacks hold rook and crows (5)
I believe the answer is:
brags
'crows' is the definition.
('brag' can be a synonym of 'crow')
'sacks hold rook' is the wordplay.
'sacks' becomes 'bags' (I've seen this in another clue).
'hold' means one lot of letters goes inside another.
'rook' becomes 'R' (chess abbreviation).
'bags' enclosing 'r' is 'BRAGS'.
'and' is the link.
(Other definitions for brags that I've seen before include "Crows" , "Boasts" .)