Lengthen unnecessarily (3)
I believe the answer is:
pad
'lengthen unnecessarily' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pad that I've seen before include "Cushion" , "In slang, one's flat or bedsitter" , "Batsman's protection" , "Floating leaf of a lily" , "Number of sheets of paper fastened together" .)