Certainly, old girl (4)
I believe the answer is:
okay
'certainly' is the definition.
(thesaurus)
'old girl' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'girl' becomes 'kay' (**).
'o'+'kay'='OKAY'
(Other definitions for okay that I've seen before include "Correct" , "Leave" , "Fairly good; approve" , "Yes; satisfactory" , "Give approval to" .)