Girl has to perform in charge of star system (8)
I believe the answer is:
'star system' is the definition.
(I've seen this before)
'girl has to perform in charge' is the wordplay.
'girl' becomes 'gal' (synonyms).
'has' says to put letters next to each other.
'to perform' becomes 'act' (acting is a kind of performing).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'of' is the link.
(Other definitions for galactic that I've seen before include "Huge - relating to a star system" , "To do with large independent systems of stars" , "Relating to, eg, the Milky Way" , "Referring to the star system" , "More than stellar" .)