Sticky stuff covering small ruler (4)
I believe the answer is:
tsar
'ruler' is the definition.
(historical term for Russian king)
'sticky stuff covering small' is the wordplay.
'sticky stuff' becomes 'tar' (tar is a sticky substance).
'covering' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tar' placed around 's' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Monarch of old Russia" , "Word derived from Caesar, used by European rulers since 1547" , "Supreme authority" , "One associated with 26 [KREMLIN]" , "Ruler finally executed" .)