A second American turned up and maintained church's authority (9)
I believe the answer is:
assurance
'authority' is the definition.
(synonyms)
'a second american turned up and maintained church's' is the wordplay.
'second american' becomes 'uss' (I can't justify this - if you can you should give a lot more credence to this answer).
'turned up' shows that the letters should be reversed in order.
'and' means one lot of letters go next to another.
'maintained' becomes 'ran' (I've seen this in another clue).
'church' becomes 'CE' (abbreviation for Church of England).
'uss' reversed gives 'ssu'.
'a'+'ssu'+'ran'+'ce'='ASSURANCE'
Can you help me to learn more?
(Other definitions for assurance that I've seen before include "what may follow life" , "Word given" , "(Life) cover" , "Confidence, composure" , "Confidence (in abilities)" .)