Goes nuts about republican, together with one that's pretentious (9)
I believe the answer is:
grandiose
'pretentious' is the definition.
(similar in meaning)
'goes nuts about republican together with one' is the wordplay.
'nuts' is an anagram indicator (nuts can mean crazy).
'about' means one lot of letters goes inside another.
'republican together' becomes 'r' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' becomes 'and'.
'one' becomes 'i' (Roman numeral).
'goes' with letters rearranged gives 'gose'.
'r'+'and'+'i'='randi'
'gose' enclosing 'randi' is 'GRANDIOSE'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for grandiose that I've seen before include "Pretentiously magnificent" , "Pretentiously impressive" , "Imposing and pompous, dressed in organdies" , "affected" , "Impressive in style" .)