De-crease? (4)
I believe the answer is:
iron
'de-crease?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iron that I've seen before include "Magnetic metallic element" , "Smooth - metal" , "medicinal preparation for the anaemic?" , "Heavy metal that rusts easily" , "Smooth things" .)