Perhaps William’s on a roll as they say, can you differentiate? (4,5)
I believe the answer is:
tell apart
'differentiate?' is the definition.
(synonyms)
'perhaps william's on a roll as they say can you' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'say' could be 'tell' (synonyms) and 'tell' is located in the answer.
The remaining letters 'part' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for tell apart that I've seen before include "See difference between" , "Rattle pal about - distinguish one from the other" .)