Trick a girl into reforming a group of stars (13)
I believe the answer is:
constellation
'a group of stars' is the definition.
(I've seen this before)
'trick a girl into reforming' is the wordplay.
'trick' becomes 'con' (I've seen this before).
'a girl' becomes 'stella' (**).
'reforming' indicates anagramming the letters.
'into' is an anagram of 'tion'.
'con'+'stella'+'tion'='CONSTELLATION'
(Other definitions for constellation that I've seen before include "Draco, perhaps" , "Twenty-five across for instance" , "Twins, perhaps" , "Group of stars" , "Formation of stars" .)