Play about a duke’s spaniel (5)
I believe the answer is:
toady
'spaniel' is the definition.
(thesaurus)
'play about a duke's' is the wordplay.
'play' becomes 'toy' (synonyms).
'about' is an insertion indicator.
'duke' becomes 'd' (abbreviation).
'a'+'d'='ad'
'toy' enclosing 'ad' is 'TOADY'.
(Other definitions for toady that I've seen before include "Person who flatters and behaves obsequiously to another" , "Obsequious type" , "crawler" , "Creep" , "Obsequious person" .)