I might need a ruler, me — a certain king? (8)
I believe the answer is:
measurer
'i might need a ruler' is the definition.
(you might use a ruler to measure something)
'me a certain king?' is the wordplay.
'certain' becomes 'sure' (synonyms).
'king?' becomes 'R' (abbreviation for rex, king in Latin).
'me'+'a'+'sure'+'r'='MEASURER'
(Other definitions for measurer that I've seen before include "Maybe tape" , "ruler" , "One with tape" , "Perhaps tape" .)