With help from the palace, call for fortification (8)
I believe the answer is:
'for fortification' is the definition.
(palisade is a kind of fortification)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for palisade that I've seen before include "paling" , "Fence of stakes or iron railings" , "Fortification" , "Stockade" , "defence work" .)