Singer's beginning to shake booty (4)
I believe the answer is:
swag
'booty' is the definition.
('swag' can be a synonym of 'booty')
'singer's beginning to shake' is the wordplay.
'beginning to' suggests taking the first letters.
'shake' becomes 'wag' ('wag' can be a synonym of 'shake').
The initial letter of 'singer' is 's'.
's'+'wag'='SWAG'
(Other definitions for swag that I've seen before include "Burglar's haul" , "Goods taken by burglar" , "pack of Aussie travellers" , "Thief's bundle" , "items stolen" .)