Old Roman emperor meets American heavy (7)
I believe the answer is:
onerous
'heavy' is the definition.
(similar in meaning)
'old roman emperor meets american' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'roman emperor' becomes 'nero' (I've seen this before).
'meets' means one lot of letters go next to another.
'american' becomes 'us' (abbreviation for United States).
'o'+'nero'+'us'='ONEROUS'
(Other definitions for onerous that I've seen before include "Involving a great deal of effort" , "weight" , "Burdensome as duty may be" , "Difficult, troublesome" , "Burdensome, weighty" .)