Nominally, would he rule doggedly? (3)

I believe the answer is:
rex
'nominally would he' is the definition.
The definition suggests a singular noun which matches the answer.
'rule doggedly?' is the wordplay.
'rule' becomes 'r' (abbreviation for rule).
'doggedly?' becomes 'ex' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'r'+'ex'='REX'
Can you help me to learn more?
(Other definitions for rex that I've seen before include "Part of official title of a king" , "See 6" , "Oedipus - -" , "One in the palace" , "King - Harrison?" .)
