They can help make a muddy estuary a bit colourful! (4)
I believe the answer is:
dyes
'they' is the definition.
Both the definition and answer are plural nouns.
Maybe you can see an association between them that I don't see?
'help make a muddy estuary a bit colourful' is the wordplay.
'DYES' is hidden within the letters.
However, I am unsure how this is indicated.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for dyes that I've seen before include "Stains" , "Colours (fabric)" , "Colour" , "turns green, possibly" , "Many a pigment" .)