Military barrier could be made by cadets, OK? (8)
I believe the answer is:
stockade
'military barrier' is the definition.
(type of defensive barrier)
'could be made by cadets ok?' is the wordplay.
'could be' is an anagram indicator.
'made by' is a charade indicator (letters next to each other) (letters written or made by another set of letters).
'cadets' after 'ok' is 'okcadets'.
'okcadets' with letters rearranged gives 'STOCKADE'.
(Other definitions for stockade that I've seen before include "It offers protection" , "defensive structure" , "Fortification" , "Fenced penal area" , "Cod steak consumed in enclosure as for military prisoners" .)