Group of stars gathering at address (7)
I believe the answer is:
Here is my best explanation:
'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 "power", "words", "Type 16 [OF SPEECH]", "A long ceremonial speech", "Power of speech", "Platform speech", "Major speech", "Making a speech", "A formal or public speech" and "Long speech".)