One burrowing underground cut down king's fortification (9)
I believe the answer is:
earthwork
'fortification' is the definition.
(earthwork is a kind of fortification)
'one burrowing underground cut down king's' is the wordplay.
'one burrowing underground' becomes 'earthworm'.
'cut down' means to remove the last letter.
'king' becomes 'k'.
'earthworm' with its last letter removed is 'earthwor'.
'earthwor'+'k'='EARTHWORK'
(Other definitions for earthwork that I've seen before include "Banking" , "Defensive bank of soil" , "Rampart made of soil" , "Bank made to defend" , "Large artificial soil bank" .)