John Travolta's beginning to get sack (4)
I believe the answer is:
loot
'sack' is the definition.
(I've seen this before)
'john travolta's beginning to' is the wordplay.
'john' becomes 'loo' (both can mean a toilet).
'beginning to' says to take the initial letters.
The initial letter of 'travolta' is 't'.
'loo'+'t'='LOOT'
'get' is the link.
(Other definitions for loot that I've seen before include "Plunder, pillage" , "familiar officer" , "Plunder and ransack" , "Haul" , "Plunder - booty" .)