The pain of being clever? (5)
I believe the answer is:
smart
I believe this clue is a double definition.
'the pain' is the first definition.
(smart is a kind of pain)
'being clever?' is the second definition.
('smart' can be similar in meaning to 'clever')
'of' acts as a link.
(Other definitions for smart that I've seen before include "Clever, ingenious" , "Clever or dapper" , "Clean and stylish" , "Fashionable" , "Elegant and stylish" .)