100 seen in backing current decree (5)
I believe the answer is:
edict
'decree' is the definition.
(synonyms)
'100 seen in backing current' is the wordplay.
'100' becomes 'C' (Roman numeral).
'seen in' indicates putting letters inside.
'backing' says the letters should be written in reverse.
'current' becomes 'tide' (sea currents).
'tide' in reverse letter order is 'edit'.
'c' put within 'edit' is 'EDICT'.
(Other definitions for edict that I've seen before include "Decree, fiat" , "Regulation" , "Cited (anag) - proclamation" , "Formal order" , "Official order" .)