Old Roman emperor seen with American heavy (7)
I believe the answer is:
onerous
'heavy' is the definition.
(similar in meaning)
'old roman emperor seen with american' is the wordplay.
'old' becomes 'o' (abbreviation).
'roman emperor' becomes 'Nero' (Roman emperor, 37-68 AD).
'seen with' is a charade indicator (letters next to each other).
'american' becomes 'US' (United States).
'o'+'nero'+'us'='ONEROUS'
(Other definitions for onerous that I've seen before include "Toilsome" , "Trying" , "Heavy, difficult to do or bear" , "Challenging" , "Burdensome as duty may be" .)