Arrange the sheets (7)
I believe the answer is:
'arrange the sheets' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for collate that I've seen before include "Compare and analyse (two or more sources)" , "Assemble in sequence" , "Gather and put in order" , "Arrange (papers)" , "Bring together and arrange" .)