Son of Rex? (6)
I believe the answer is:
prince
This might be a double definition.
'son' is the first definition.
(I have seen 'Royal son ' mean 'prince' so perhaps 'son' could also mean 'prince')
'rex?' is the second definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?
'of' is the link.
Can you help me to learn more?
(Other definitions for prince that I've seen before include "Son of a royal house" , "Panto character" , "Royal personage" , "Son of a sovereign" , "Pincer movement around the royal" .)