Singer's beginning to shake booty (4)
I believe the answer is:
swag
'booty' is the definition.
(I know that booty can be written as swag)
'singer's beginning to shake' is the wordplay.
'beginning to' suggests taking the first letters.
'shake' becomes 'wag' (synonyms).
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" .)