Say earl is retreating seeing military blockade (5)
I believe the answer is:
siege
'military blockade' is the definition.
(I know that military blockade can be written as siege)
'say earl is retreating' is the wordplay.
'say' becomes 'eg' (both can mean for example).
'earl' becomes 'e'.
'retreating' says the letters should be written backwards.
'eg'+'e'+'is'='egeis'
'egeis' back-to-front is 'SIEGE'.
'seeing' is the link.
(Other definitions for siege that I've seen before include "seat" , "Offence in which fortified place is surrounded" , "Military operation surrounding a town" , "Besetting" , "Blockade of a town or fortress" .)