Wise men, not completely imaginary (4)
I believe the answer is:
magi
'wise men' is the definition.
(the three wise men from the nativity)
'not completely imaginary' is the wordplay.
'not completely' indicates the answer is hidden within the clue.
'MAGI' is hidden in the letters of 'imaginary'.
(Other definitions for magi that I've seen before include "Sorcerers in ancient times" , "Biblical wise men" , "The Three Kings" , "Eastern men of lore; Epiphany figures" , "members of priestly caste" .)