Group of stars gathering at address (7)
I believe the answer is:
oration
'address' is the definition.
(oration is a kind of address)
'group of stars gathering at' is the wordplay.
'group of stars' becomes 'orion' (I've seen this before).
'gathering' means one lot of letters goes inside another.
'orion' enclosing 'at' is 'ORATION'.
(Other definitions for oration that I've seen before include "A formal or public speech" , "An eloquent speech" , "Making a speech" , "A long ceremonial speech" , "A formal or ceremonial speech" .)