… as a result of American engaging in various topics (4,5)
I believe the answer is:
ipso facto
'as a result' is the definition.
(I've seen this before)
'of american engaging in various topics' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'engaging in' is an insertion indicator (I've seen this in other clues).
'various' is an anagram indicator.
'of'+'a'='ofa'
'topics' anagrammed gives 'ipscto'.
'ofa' inserted into 'ipscto' is 'IPSO-FACTO'.
(Other definitions for ipso facto that I've seen before include "By that very deed" , "For that precise reason" , "Thereby (Latin)" , "as a result" , "By that very act (Lat.)" .)