Fantastic being regularly well-off (3)
I believe the answer is:
elf
'fantastic being' is the definition.
'regularly well-off' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'well'+'off'='welloff'
The alternating letters of 'welloff' are 'ELF'.
(Other definitions for elf that I've seen before include "Imaginary small being of selfish kind" , "Santa's helper" , "Fairy-like creature" , "Hobgoblin" , "Diminutive folklore creature" .)