Getting rook inside sack is a game (4)
I believe the answer is:
brag
'game' is the definition.
(I've seen this before)
'getting rook inside sack' is the wordplay.
'getting' indicates putting letters inside.
'rook' becomes 'R' (chess abbreviation).
'inside sack' becomes 'bag' (I've seen this before. I am not sure about the 'inside' bit.).
'r' going inside 'bag' is 'BRAG'.
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for brag that I've seen before include "Boast, crow" , "Boast ? card-game" , "Shoot a line" , "Boast about garb" , "Card game, like poker" .)