It's of the highest importance, because a few of them have gone west with tyrannical leader (8)
I believe the answer is:
foremost
'it's' is the definition.
I can't judge whether this defines the answer.
'the highest importance because a few of them have gone west with tyrannical leader' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'because' could be 'for' (I've seen this before) and 'for' is found within the answer.
'them' could be 'em' (abbreviation) and 'em' is found within the answer.
The remaining letters 'os' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for foremost that I've seen before include "First in time, place or order" , "Most prominent" , "premier" , "Pre-eminent" , "First in rank" .)