Heavy, heavy blow above American boxing ring (7)
I believe the answer is:
onerous
'heavy' is the definition.
(similar in meaning)
'heavy blow above american boxing ring' is the wordplay.
'heavy blow' becomes 'oner' (I've seen this before).
'above' says to put letters next to each other (in a down clue, some letters go above others).
'american' becomes 'US' (United States).
'boxing' is an insertion indicator ('box' can mean contain, as in to put in a box).
'ring' becomes 'O' (letter resembling a ring).
'oner'+'us'='onerus'
'onerus' going around 'o' is 'ONEROUS'.
(Other definitions for onerous that I've seen before include "'Heavy, burdensome (7)'" , "Heavy, difficult to do or bear" , "Involving much effort" , "'Burdensome, not easily borne (7)'" , "Oppressive, difficult to bear" .)