Species of kipper under water plants essentially hidden (3,7)
I believe the answer is:
red snapper
'species' is the definition.
'kipper under water plants essentially hidden' is the wordplay.
'kipper' becomes 'napper'.
'under' means one lot of letters go next to another.
'water plants' becomes 'reeds' (I've seen this before).
'essentially hidden' means to remove the middle letters.
'reeds' with its centre taken out is 'reds'.
'napper' put after 'reds' is 'RED SNAPPER'.
'of' acts as a link.
(Other definitions for red snapper that I've seen before include "swimmer" , "Marine food fish" .)