One using ruler from Middle East, a certain king (8)
I believe the answer is:
measurer
'one using ruler' is the definition.
(someone measuring things with a ruler)
'middle east a certain king' is the wordplay.
'middle east' becomes 'me'.
'certain' becomes 'sure' (similar in meaning).
'king' becomes 'r' (r is abbreviation for rex).
'me'+'a'+'sure'+'r'='MEASURER'
'from' acts as a link.
(Other definitions for measurer that I've seen before include "ruler" , "One with tape" , "Perhaps tape" , "Maybe tape" .)