Desk in small room briefly overturned on seabird (7)
I believe the answer is:
lectern
'desk' is the definition.
(I've seen this in another clue)
'small room briefly overturned on seabird' is the wordplay.
'small room' becomes 'cell' (eg a prison cell).
'briefly' means to remove the last letter.
'overturned' says the letters should be written backwards.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'seabird' becomes 'tern' (tern is a kind of seabird).
'cell' with its final letter taken off is 'cel'.
'cel' reversed gives 'lec'.
'lec'+'tern'='LECTERN'
'in' acts as a link.
(Other definitions for lectern that I've seen before include "Slanted reading desk" , "Bible stand" , "Speaker's stand" , "Desk for a standing reader" , "Support for bible in church" .)