Boisterous males casing knight's estate (5)
I believe the answer is:
lands
'estate' is the definition.
(lands can mean an estate)
'boisterous males casing knight's' is the wordplay.
'boisterous males' becomes 'lads' (lad can mean a young male).
'casing' is an insertion indicator (I've seen this in other clues).
'knight' becomes 'N' (chess abbreviation).
'lads' going around 'n' is 'LANDS'.
(Other definitions for lands that I've seen before include "See 21" , "Obtains" , "Comes ashore" , "Alights" .)