Begin fantastically with minor (6)
I believe the answer is:
infant
'minor' is the definition.
(thesaurus)
'begin fantastically with' is the wordplay.
'with' indicates the answer is hidden within the clue.
'INFANT' is hidden within 'begin fantastically'.
(Other definitions for infant that I've seen before include "Thing in early stage; minor (legal)" , "Babe" , "Tot" , "Rudimentary" , "Juvenile" .)