Difficult, old emperor coming to America (7)
I believe the answer is:
onerous
'difficult' is the definition.
(thesaurus)
'old emperor coming to america' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'emperor' becomes 'nero' (Roman emperor).
'coming to' means one lot of letters go next to another.
'america' becomes 'us' (abbreviation for the United States).
'o'+'nero'+'us'='ONEROUS'
(Other definitions for onerous that I've seen before include "Involving much effort" , "Burdensome, heavy kind of duty" , "Effortful" , "Involving much work" , "'Heavy, burdensome (7)'" .)