Order cathead to be immersed in the rising tide (5)
I believe the answer is:
edict
'order' is the definition.
(I know that order can be written as edict)
'cathead to be immersed in the rising tide' is the wordplay.
'cathead' becomes 'c' (head letter of 'cat').
'to be immersed in' is an insertion indicator.
'the rising' is a reversal indicator (in a down clue, letters go up).
'tide' reversed gives 'edit'.
'c' going inside 'edit' is 'EDICT'.
(Other definitions for edict that I've seen before include "Formal order" , "Formal announcement" , "Pronouncement" , "Injunction" , "Legally-binding order" .)