Crossword Genius

Certainly, old girl (4)

Ross

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" .)

I've seen this clue in The Times.
Want a hint initially instead of a full solution? Install my app