At bottom, they're supportive (5)
I believe the answer is:
seats
'at' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'bottom they're supportive' is the wordplay.
'bottom' becomes 'sea bottom' (sea bottom is a kind of bottom).
'they're supportive' becomes 'ts' (this could be a standard abbreviation I've not previously seen).
'sea'+'ts'='SEATS'
Can you help me to learn more?
(Other definitions for seats that I've seen before include "Chairs, stools" , "Parliamentary constituencies" .)