On reflection I think no American is threatening (7)
I believe the answer is:
ominous
'is threatening' is the definition.
(synonyms)
'on reflection i think no american' is the wordplay.
'on reflection' says the letters should be written in reverse.
'i think' becomes 'imo' (abbreviation for 'in my opinion').
'american' becomes 'US' (abbreviation for United States).
'imo' back-to-front is 'omi'.
'omi'+'no'+'us'='OMINOUS'
(Other definitions for ominous that I've seen before include "Not looking good, dark clouds ahead" , "Threatening, foreshadowing something bad" , "Boding ill" , "'Threatening, foreshadowing tragedy (7)'" , "Threatening, looks bad" .)