As soon as possible, couple cleared out (4)
I believe the answer is:
once
'as soon as' is the definition.
(once can mean immediately after)
'possible couple cleared out' is the wordplay.
'possible' becomes 'on' ('not on' can mean unacceptable or not possible).
'cleared out' suggests removing the centre.
'couple' with its middle removed is 'ce'.
'on'+'ce'='ONCE'
(Other definitions for once that I've seen before include "in bygone days" , "Cone (anag.)" , "In former times" , "Singly" , "Formerly, at one time" .)