Witness seized by Ethelred -- lo, he begs to get up (8)
I believe the answer is:
beholder
'witness' is the definition.
(I've seen this before)
'ethelred lo he begs to get up' is the wordplay.
'to get' says the answer is hidden in the clue (I've seen 'gets' mean this).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'redloheb' can be found hidden inside 'ethelred lo he begs'.
'redloheb' backwards is 'BEHOLDER'.
'seized by' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for beholder that I've seen before include "there's beauty in her eyes" , "Beauty is said to be in this eye" , "Witness" , "Observer" , "One sees" .)