Blockade in school that’s from behind, say (5)
I believe the answer is:
siege
'blockade' is the definition.
(I know that siege is a type of blockade)
'school that's from behind say' is the wordplay.
'school' becomes 'S'.
'that's' becomes 'ie'.
'from behind' says the letters should be written in reverse.
'say' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'eg' back-to-front is 'ge'.
's'+'ie'+'ge'='SIEGE'
'in' acts as a link.
(Other definitions for siege that I've seen before include "Military encirclement" , "Blockade of a fortified place" , "Military operation ringing a town" , "Operation to capture a fort, for example" , "Starving-out operation" .)