Difficult to be sly (6)
I believe the answer is:
tricky
I believe this is a double definition.
'difficult' is the first definition.
(similar in meaning)
'be sly' is the second definition.
(synonyms)
'to' acts as a link.
(Other definitions for tricky that I've seen before include "Awkward - dishonest" , "Difficult; sly" , "Problematic - deceitful" , "Crafty or difficult" , "Crafty, cunnning" .)