Bloody lavatory emptied once in a blue moon (6)
I believe the answer is:
rarely
'once in a blue moon' is the definition.
(I've seen this before)
'bloody lavatory emptied' is the wordplay.
'bloody' becomes 'rare' (I've seen this before).
'emptied' suggests removing the centre.
'lavatory' with its centre removed is 'ly'.
'rare'+'ly'='RARELY'
(Other definitions for rarely that I've seen before include "when hundred-to-one horse wins?" , "Not done or occurring very often" , "Gas used in fluorescent signs" , "once in a blue moon" , "Not very often, seldom in fact" .)