McKellen who played Gandalf (3)
I believe the answer is:
'mckellen who played gandalf' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ian that I've seen before include "Boy's name (Scottish?)" , "Scotsman?" , "... Fleming, author of the Bond books" , "Albert Beale's first name in EastEnders" , "The familiar Mr Paisley" .)