Sea creature — clownfish — seen in an estuary's mouth (7)
I believe the answer is:
anemone
'sea creature' is the definition.
(I've seen this in another clue)
'clownfish seen in an estuary's mouth' is the wordplay.
'clownfish' becomes 'nemo' (from the film Finding Nemo).
'seen in' is an insertion indicator.
'mouth' suggests taking the first letters (mouth can mean something's entrance).
The initial letter of 'estuarys' is 'e'.
'nemo' put into 'an' is 'anemon'.
'anemon'+'e'='ANEMONE'
(Other definitions for anemone that I've seen before include "Kind of buttercup with white or coloured flowers" , "Plant with brightly coloured flowers - found also in sea" , "Woodland plant - name one (anag)" , "Plant; sea creature" , "Plant with bright flowers" .)