Seat that's too low? Not entirely (5)
I believe the answer is:
stool
'seat' is the definition.
(I know that seat can be written as stool)
'that's too low? not entirely' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'STOOL' can be found hidden inside 'thats too low'.
(Other definitions for stool that I've seen before include "Three-legged backless seat" , "Motion" , "Backless chair" , "Small seat" , "Simple seat" .)