A regular poser no doubt (7,5)
I believe the answer is:
artists model
'no doubt' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'a regular poser' is the wordplay.
'a regular' becomes 'artists' (I can't justify this - if you can you should believe this answer much more).
'poser' becomes 'model' (e.g. a model for an artist - someone who poses).
'artists'+'model'='ARTISTS MODEL'
Can you help me to learn more?
(Another definition for artists model that I've seen is " Sitter for painter".)