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' indicates a hidden word.
'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" .)