New commercial includes excellent, fantastic girl (5)
I believe the answer is:
naiad
'fantastic girl' is the definition.
(type of spirit in Greek myth)
'new commercial includes excellent' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'commercial' becomes 'ad' (abbreviation for advertisement).
'includes' means one lot of letters goes inside another.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'n'+'ad'='nad'
'nad' enclosing 'ai' is 'NAIAD'.
(Other definitions for naiad that I've seen before include "Diana (anag.)" , "Spring nymph" , "Aquatic larva of a dragonfly" , "River nymph" , "Nymph of lakes, springs and rivers" .)