Temporary stable lad unseated here (8,4)
I believe the answer is:
standing room
'unseated here' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'temporary stable lad' is the wordplay.
'temporary' becomes 'stand-in'.
'stable lad' becomes 'groom' (I've seen this before).
'standin'+'groom'='STANDING-ROOM'
Can you help me to learn more?
(Other definitions for standing room that I've seen before include "Space for upright people?" , "Where people won't five down [SIT DOWN]" , "No seats left (on bus)" , "No place on bench" , "nowhere to sit" .)