Round-backed beast? Certainly (4)
I believe the answer is:
okay
'certainly' is the definition.
(thesaurus)
'round-backed beast?' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'backed' says the letters should be written backwards.
'beast?' becomes 'yak' (I have seen 'Beast of burden ' mean 'yak' so perhaps 'beast' could also mean 'yak').
'yak' back-to-front is 'kay'.
'o'+'kay'='OKAY'
(Other definitions for okay that I've seen before include "Leave" , "Not terrific but acceptable" , "Fairly good; approve" , "Authorise" , "Agreed!" .)