Herons flying over French island's receding littoral (9)
I believe the answer is:
'littoral' is the definition.
'herons flying over french island's receding' is the wordplay.
'flying' indicates an anagram.
'over' indicates putting letters inside.
'french island' becomes 'ile' (French for 'island').
'receding' shows that the letters should be reversed in order.
'ile' reversed gives 'eli'.
'herons' is an anagram of 'shorne'.
'shorne' placed around 'eli' is 'SHORELINE'.
(Other definitions for shoreline that I've seen before include "Where the sea meets land" , "Meeting point of land and sea" , "Land's edge" , "Coastal strip" .)