Heavy old Roman emperor bust uncovered (7)
I believe the answer is:
onerous
'heavy' is the definition.
('onerous' can be similar in meaning to 'heavy')
'old roman emperor bust uncovered' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'roman emperor' becomes 'nero' (I've seen this before).
'uncovered' indicates the central letters.
The centre of 'bust' is 'us'.
'o'+'nero'+'us'='ONEROUS'
(Other definitions for onerous that I've seen before include "No euros? That's burdensome" , "Involving a great deal of effort" , "Challenging" , "Difficult, troublesome" , "Burdensome, heavy kind of duty" .)