Pool used up in ceremony (4)
I believe the answer is:
mere
'pool' is the definition.
(mere can mean a lake)
'up in ceremony' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' indicates a hidden word.
'erem' can be found hidden inside 'ceremony'.
'erem' reversed gives 'MERE'.
'used' acts as a link.
(Other definitions for mere that I've seen before include "Unimpressive" , "Lake; Wiltshire town" , "Pool -- pure and simple" , "Nothing more than a small pool" , "Nothing more than a small pond" .)