Certainly a variety of tuberose (2,2,4)
I believe the answer is:
to be sure
'certainly' is the definition.
(thesaurus)
'a variety of tuberose' is the wordplay.
'a variety of' is an anagram indicator.
'tuberose' with letters rearranged gives 'TO BE SURE'.
(Other definitions for to be sure that I've seen before include "Undoubtedly" , "I admit" , "Of course, certainly" , "Without doubt" , "Admittedly" .)