One who elects to veto royal amendment (5)
I believe the answer is:
voter
'one who elects' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'veto royal amendment' is the wordplay.
'royal' becomes 'r' (common abbreviation e.g. RM - Royal Marine).
'amendment' is an anagram indicator.
'veto'+'r'='vetor'
'vetor' with letters rearranged gives 'VOTER'.
'to' is the link.
Can you help me to learn more?
(Other definitions for voter that I've seen before include "One of the selectors" , "Elector" , "Lively dances (esp. Irish)" , "supporter of politician" , "Constituent" .)