After fish, do some shopping for marine plant (8)
I believe the answer is:
eelgrass
'marine plant' is the definition.
(I've seen this before)
'after fish do some shopping' is the wordplay.
'after' means one lot of letters go next to another.
'fish' becomes 'eel' (eel is a type of fish).
'do' becomes 'gras' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'some' says to take the initial letters.
The initial letter of 'shopping' is 's'.
'eel'+'gras' is 'eelgras'.
'eelgras'+'s'='EELGRASS'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for eelgrass that I've seen before include "Marine plant" , "Flowering plant of pondweed family" , "Submerged aquatic plant with ribbonlike leaves" .)