Old Emperor taking on America, implying a lot of work (7)
I believe the answer is:
onerous
'implying a lot of work' is the definition.
(onerous means difficult)
'old emperor taking on america' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'emperor' becomes 'nero' (Roman emperor).
'taking on' is a charade indicator (letters next to each other).
'america' becomes 'US' (United States).
'o'+'nero'+'us'='ONEROUS'
(Other definitions for onerous that I've seen before include "Involving a great deal of effort" , "'Heavy, burdensome (7)'" , "Involving much effort" , "Burdensome, demanding" , "Involving much work" .)