Yes, the ruling head's title! (3)
I believe the answer is:
sir
'title' is the definition.
(I've seen this before)
'yes the ruling head's' is the wordplay.
'yes' becomes 'si' (I am not sure about this - if you are sure you should believe this answer much more).
'head's' says to take the initial letters (I've seen 'heading' mean this (head could mean the top letter)).
The first letter of 'ruling' is 'r'.
'si'+'r'='SIR'
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Honourable title" , "Baronet's title" , "Way to address gentleman" , "Form of address for a knight" , "Feted fellow" .)