Young woman launching a festival (4)
I believe the answer is:
gala
'festival' is the definition.
(I've seen this before)
'young woman launching a' is the wordplay.
'young woman' becomes 'gal' (synonyms).
'launching' means one lot of letters go next to another (launch can mean to begin).
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "Festive celebration" , "Festival" , "fair" , "sporting event" , "Public party" .)