Presumably Express being to the right of this setter's challenging (1,7)
I believe the answer is:
i daresay
'presumably' is the definition.
'express being to the right of this setter's challenging' is the wordplay.
'express' becomes 'say' (I've seen this before).
'being to the right of' means one lot of letters go next to another.
'this setter' becomes 'i' (the person that set the crossword).
'challenging' becomes 'dare' (I can't justify this - if you can you should give a lot more credence to this answer).
'i'+'dare'='idare'
'say' after 'idare' is 'I DARESAY'.
(Other definitions for i daresay that I've seen before include "Maybe" , "Probably" , "Methinks it's likely that" .)