Brawny American couple oddly beset by ruin (8)
I believe the answer is:
muscular
'brawny' is the definition.
(synonyms)
'american couple oddly beset by ruin' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'ruin' becomes 'mar' (I've seen this before).
The alternating letters of 'couple' are 'cul'.
'us'+'cul'='uscul'
'uscul' put within 'mar' is 'MUSCULAR'.
(Other definitions for muscular that I've seen before include "Powerful, strong" , "Strapping" , "Brawny, strong" , "Brawny, sinewy" , "'Brawny, powerfully built (8)'" .)