Rests on the bottom (4)
I believe the answer is:
sits
'rests' is the definition.
(I've seen this in another clue)
'on the bottom' is the wordplay.
'on' indicates putting letters inside (as in clothing 'on' a person).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'bottom' becomes 'sis' (I can't explain this - if you can you should believe this answer much more).
't' put into 'sis' is 'SITS'.
Can you help me to learn more?
(Other definitions for sits that I've seen before include "Poses for portrait" , "Perches" , "Takes a seat for the painter" , "Takes an exam" , "Poses for a painter" .)