Mattress part (4)
I believe the answer is:
coil
'mattress part' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for coil that I've seen before include "Something wound" , "Convolute" , "Curl, loop" , "Wind (ropes) - gather (hair)" , "Wind round and round in loops" .)