Being humble is unexceptional (6)
I believe the answer is:
I believe this is a double definition.
'being humble' is the first definition.
('modest' can be similar in meaning to 'humble')
'unexceptional' is the second definition.
'is' is the link.
(Other definitions for modest that I've seen before include "Inexpensive" , "Unassuming or unprententious" , "Humble, unpretentious" , "Unassuming; limited" , "Prudent" .)