Supreme leader of men in land unknown (8)
I believe the answer is:
almighty
'supreme' is the definition.
(I've seen this before)
'leader of men in land unknown' is the wordplay.
'leader of' suggests taking the first letters.
'in' is an insertion indicator.
'land' becomes 'alight' (alighting is a kind of landing).
'unknown' becomes 'y' (algebra).
The initial letter of 'men' is 'm'.
'm' put within 'alight' is 'almight'.
'almight'+'y'='ALMIGHTY'
(Other definitions for almighty that I've seen before include "Supreme" , "Supremely powerful" , "Huge" , "All-powerful - like God" , "Excessive" .)