Certainly a lowish bid by girl (2,5)
I believe the answer is:
in truth
'certainly' is the definition.
(thesaurus)
'a lowish bid by girl' is the wordplay.
'a lowish bid' becomes 'int' (I can't explain this - if you can you should believe this answer much more).
'by' says to put letters next to each other.
'girl' becomes 'Ruth' (girl's name).
'int'+'ruth'='IN TRUTH'
Can you help me to learn more?
(Other definitions for in truth that I've seen before include "As a matter of fact" , "Really" , "Actually" , "To be honest" , "Honestly" .)