Grass skirts with jumper in an estuary (10)
I believe the answer is:
sandhopper
'jumper in an estuary' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'grass skirts with' is the wordplay.
'grass' becomes 'shopper' (I am not sure about this - if you are sure you should believe this answer much more).
'skirts' indicates putting letters inside (skirt can mean to go around).
'with' becomes 'and'.
'shopper' going around 'and' is 'SANDHOPPER'.
Can you help me to learn more?
(Other definitions for sandhopper that I've seen before include "jumper for the beach?" , "tiny crustacean" .)