Unknown number removed from opposite side by a clear-out (5)
I believe the answer is:
enema
'clear-out' is the definition.
(medical procedure)
'unknown number removed from opposite side by a' is the wordplay.
'unknown number' becomes 'y' (I've seen this in other clues).
'removed from' is a deletion indicator.
'opposite side' becomes 'enemy' (I've seen this in another clue).
'by' says to put letters next to each other.
'enemy' with 'y' taken away is 'enem'.
'enem'+'a'='ENEMA'
(Other definitions for enema that I've seen before include "Stern action from nurse perhaps" , "Rectal medicine" , "Procedure for vacating the bowels" , "process to ensure evacuation" , "fundamental procedure?" .)