Fantastic man, 'lion' so-called (7)
I believe the answer is:
nominal
'so-called' is the definition.
(thesaurus)
'fantastic man lion' is the wordplay.
'fantastic' indicates an anagram.
'man'+'lion'='manlion'
'manlion' with letters rearranged gives 'NOMINAL'.
(Other definitions for nominal that I've seen before include "So-called, but not in reality" , "virtually nothing" , "Relating to names" , "Minor" , "Insignificantly small - token" .)