Short song a girl’s written about an alien (7)
I believe the answer is:
arietta
'short song' is the definition.
(a short aria)
'a girl's written about an alien' is the wordplay.
'girl' becomes 'rita' (woman's name).
'written about' indicates putting letters inside.
'an alien' becomes 'et' (extra terrestrial).
'rita' enclosing 'et' is 'rietta'.
'a'+'rietta'='ARIETTA'
(Other definitions for arietta that I've seen before include "number from opera" , "Little aria" , "Short operatic number" , "Short operatic air" , "Little song" .)