Marine plant producing wonder in beginning (7)
I believe the answer is:
seaweed
'marine plant' is the definition.
(I've seen this before)
'wonder in beginning' is the wordplay.
'wonder' becomes 'awe' (awe is a kind of wonder).
'in' indicates putting letters inside.
'beginning' becomes 'seed' (seed is a kind of beginning**).
'awe' put within 'seed' is 'SEAWEED'.
'producing' is the link.
(Other definitions for seaweed that I've seen before include "Eg, kelp" , "water plant" , "Coastal plants" , "Marine plant" , "Marine alga" .)