Obsessive behaviour, say, to replace leader of country (8)
I believe the answer is:
egomania
'obsessive behaviour' is the definition.
The definition suggests a singular noun which matches the answer.
'say to replace leader of country' is the wordplay.
I cannot quite see how this works, but
'say' could be 'eg' (short for 'exempli gratia', 'for example' in Latin) and 'eg' is found within the answer.
'of' could be 'o' and 'o' is located in the answer.
The remaining letters 'mania' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for egomania that I've seen before include "Extreme self-centredness" , "Obsessive self-regard" , "Great self-absorption" , "having no thought for others" , "Obsessive self-interest" .)