Restyle Elvis occasionally, coming back lithe ... (6)
I believe the answer is:
svelte
'lithe' is the definition.
(synonyms)
'restyle elvis occasionally coming back' is the wordplay.
'occasionally' means one should take alternating letters.
'coming back' shows that the letters should be reversed in order.
'restyle'+'elvis'='restyleelvis'
The alternate letters of 'restyleelvis' are 'etlevs'.
'etlevs' reversed gives 'SVELTE'.
(Other definitions for svelte that I've seen before include "Attractively slim" , "Gracefully slender" , "free" , "Gracefully slim" , "Slender - refined" .)