One standing in for royal pair, earnest man discharging seconds (6,6)
I believe the answer is:
prince regent
'one standing in for royal pair earnest man discharging seconds' is the definition.
The definition suggests a singular noun which matches the answer.
'one standing in for royal' is the wordplay.
'one standing in' becomes 'regent' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' means one lot of letters go next to another (I've seen this in other clues).
'royal' becomes 'prince' (prince is a kind of royal).
'regent' put after 'prince' is 'PRINCE REGENT'.
Can you help me to learn more?
(Other definitions for prince regent that I've seen before include "As George IV was during eighteen eleven - eighteen twenty" , "George IV, before he became King" , "The future George IV in 1811-20" , "George III's son was one such" .)