Cipher once used in Belize routinely (4)
I believe the answer is:
zero
'cipher' is the definition.
(I know that cipher can be written as zero)
'used in belize routinely' is the wordplay.
'used in' indicates the answer is hidden within the clue.
'ZERO' is hidden in the letters of 'belize routinely'.
'once' is the link.
(Other definitions for zero that I've seen before include "Naught" , "Love" , "Rock bottom" , "Zilch" , "Nothing; freezing point (in degrees centigrade)" .)