Bear up, whether great or little? (13)
I believe the answer is:
constellation
'bear' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I can't explain the rest of the clue.
(Other definitions for constellation that I've seen before include "Star group" , "Twins, perhaps" , "Cassiopeia, for example" , "Twenty-five across for instance" , "Formation of stars" .)