"Through the Looking-Glass" character (10)
I believe the answer is:
tweedledum
'through the looking-glass character' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tweedledum that I've seen before include "Twin spoiling for a fight" , "He accused his brother of spoiling his rattle" , "Twin of 9 dn" , "The one who alleged that the other was a rattle spoiler" , "One of Carroll's twins" .)