American, one in new cult existing in reality (6)
I believe the answer is:
actual
'existing in reality' is the definition.
(I've seen this before)
'american one in new cult' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'one' becomes 'a'.
'in' means one lot of letters goes inside another.
'new' indicates an anagram.
'cult' with letters rearranged gives 'ctul'.
'a' placed inside 'ctul' is 'ctual'.
'a'+'ctual'='ACTUAL'
(Other definitions for actual that I've seen before include "De facto" , "Real, existing in fact" , "Existing in reality" , "Really existing" , "Real and existing (6)" .)