Celestial model peer is reassembling across border (9)
I believe the answer is:
ephemeris
'celestial model' is the definition.
The definition suggests a singular noun which matches the answer.
'peer is reassembling across border' is the wordplay.
'reassembling' is an anagram indicator.
'across' indicates putting letters inside.
'border' becomes 'hem' (I've seen this before).
'peer'+'is'='peeris'
'peeris' is an anagram of 'eperis'.
'eperis' going around 'hem' is 'EPHEMERIS'.
Can you help me to learn more?
(Other definitions for ephemeris that I've seen before include "Astronomical publication" , "Stars' table" , "Guide to Galaxy?" , "almanac" , "navigational aid?" .)