Marine algae filling children with wonder (7)
I believe the answer is:
'marine algae' is the definition.
(I've seen this before)
'children with wonder' is the wordplay.
'children' becomes 'seed' (I've seen this before).
'with' indicates putting letters inside.
'wonder' becomes 'awe' (awe is a kind of wonder).
'seed' enclosing 'awe' is 'SEAWEED'.
'filling' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
(Other definitions for seaweed that I've seen before include "Marine growth" , "We eased (anag)" , "What's seen at the coast" , "Marine algae" , "Principal ingredient of laverbread" .)