Mysterious male trapped in perverse union with American (8)
I believe the answer is:
numinous
'mysterious' is the definition.
(thesaurus)
'male trapped in perverse union with american' is the wordplay.
'male' becomes 'm' (common abbreviation).
'trapped in' is an insertion indicator.
'perverse' is an anagram indicator.
'with' means one lot of letters go next to another.
'american' becomes 'US' (abbreviation for United States).
'union' is an anagram of 'nuino'.
'm' placed within 'nuino' is 'numino'.
'numino'+'us'='NUMINOUS'
(Another definition for numinous that I've seen is " Sum union up in spiritual way".)