Boast keeping rook in sack (4)
I believe the answer is:
brag
'boast' is the definition.
(I know that boast can be written as brag)
'rook in sack' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'in' indicates putting letters inside.
'sack' becomes 'bag' (I've seen this before).
'r' put into 'bag' is 'BRAG'.
'keeping' acts as a link.
(Other definitions for brag that I've seen before include "Idle talk" , "Card game - boast" , "popular with 15 [EGOTIST] , no doubt" , "boast; form of poker" , "Shoot a line" .)