English youth cutting borders in Florida marshes (10)
I believe the answer is:
everglades
'florida marshes' is the definition.
(Everglades National Park in Florida)
'english youth cutting borders' is the wordplay.
'english' becomes 'E' (abbreviation).
'youth' becomes 'lad' (lad means a young man).
'cutting' is an insertion indicator (some letters cut their way into a word).
'borders' becomes 'verges' (verging is a kind of bordering).
'lad' inserted inside 'verges' is 'verglades'.
'e'+'verglades'='EVERGLADES'
'in' is the link.
(Other definitions for everglades that I've seen before include "marshy regions" , "US national park" , "Florida subtropical wilderness" .)