Giant makes even longer bed (4)
I believe the answer is:
ogre
'giant' is the definition.
(I know that ogre is a type of giant)
'even longer bed' is the wordplay.
'even' means one should take alternating letters (letters in even-numbered positions are used).
'longer'+'bed'='longerbed'
The alternate letters of 'longerbed' are 'OGRE'.
'makes' acts as a link.
(Other definitions for ogre that I've seen before include "Monstrous person" , "Giant of folklore who liked to eat humans" , "Monster, troll" , "Terrifying man" , "Grim(m) beast?" .)