Old writer yawning (4)
I believe the answer is:
open
'yawning' is the definition.
(similar in meaning)
'old writer' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'writer' becomes 'pen' (something which writes).
'o'+'pen'='OPEN'
(Other definitions for open that I've seen before include "Not closed; set going" , "Overt" , "Liberal" , "'Frank, not secret (4)'" , "Agape" .)