It is on board, is on a seat (4)

I believe the answer is:
sits
'a seat' is the definition.
The definition and answer are not the same part of speech.
'it is on board is on' is the wordplay.
'is on board' becomes 'ss' (I've seen this in other clues).
'is on' indicates putting letters inside (as in clothing 'on' a person).
'it' inserted into 'ss' is 'SITS'.
Can you help me to learn more?
(Other definitions for sits that I've seen before include "Poses for a painter" , "Takes a seat for the painter" , "Perches" , "Takes an exam" , "Poses for portrait" .)
