Comply with old Ottoman ruler (4)
I believe the answer is:
obey
'comply with' is the definition.
(I've seen this before)
'old ottoman ruler' is the wordplay.
'old' becomes 'o' (abbreviation).
'ottoman ruler' becomes 'bey' (I've seen this in another clue).
'o'+'bey'='OBEY'
(Other definitions for obey that I've seen before include "Accept instructions" , "Be ruled by" , "Follow the instructions" , "Follow orders" , "Carry out (an order)" .)