Troublesome orange leader: emperor of America (7)
I believe the answer is:
onerous
'troublesome' is the definition.
(thesaurus)
'orange leader emperor of america' is the wordplay.
'leader' indicates taking the first letters.
'emperor' becomes 'nero' (Roman emperor).
'of america' becomes 'us' ('US' can be an adjective meaning 'of America').
The initial letter of 'orange' is 'o'.
'o'+'nero'+'us'='ONEROUS'
(Other definitions for onerous that I've seen before include "Involving a great deal of effort" , "Effortful" , "Burdensome, weighty" , "Burdensome, demanding" , "Toilsome" .)