Instructed to include seal to prevent entry (8)
I believe the answer is:
blockade
'prevent entry' is the definition.
(to blockade a place is to seal it off)
'instructed to include seal' is the wordplay.
'instructed' becomes 'bade' (past tense of 'bid').
'to include' means one lot of letters goes inside another.
'seal' becomes 'lock' (seal can mean to lock up).
'bade' placed around 'lock' is 'BLOCKADE'.
'to' is the link.
(Other definitions for blockade that I've seen before include "Besiege" , "Cut off" , "The sealing off a place to prevent entry or exist" , "Seal off (a place)" , "Physical prevention of access" .)