Two in costume metamorphose in plays (9,5)
I believe the answer is:
pantomime horse
'two in costume' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'metamorphose in plays' is the wordplay.
'plays' indicates an anagram.
'metamorphose'+'in'='metamorphosein'
'metamorphosein' with letters rearranged gives 'PANTOMIME HORSE'.
Can you help me to learn more?
(Other definitions for pantomime horse that I've seen before include "Christmas show character" , "Costume for two performers" , "Christmas double act" .)