Bed size smaller than full (4)
I believe the answer is:
twin
'bed size smaller than full' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for twin that I've seen before include "This set is matching jumper and cardigan" , "Make association" , "Very much like" , "One of two conceived simultaneously" , "One hotel room" .)