Threaten one copper trapped in fix (6)
I believe the answer is:
impend
'threaten' is the definition.
(thesaurus)
'one copper trapped in fix' is the wordplay.
'one' becomes 'i' (Roman numeral).
'copper' becomes 'p'.
'trapped in' means one lot of letters goes inside another.
'fix' becomes 'mend' ('mend' can be a synonym of 'fix'**).
'p' placed inside 'mend' is 'mpend'.
'i'+'mpend'='IMPEND'
(Other definitions for impend that I've seen before include "(Of a bad thing) loom" , "Become imminent" , "Be imminent or about to happen" , "Be looming" , "Threaten; approach" .)