Tough, hard-hearted couple, American (7)
I believe the answer is:
arduous
'tough' is the definition.
(I've seen this before)
'hard-hearted couple american' is the wordplay.
'hearted' means to look at the middle letters.
'couple' becomes 'duo' (synonyms).
'american' becomes 'us' (abbreviation for United States).
The middle of 'hard' is 'ar'.
'ar'+'duo'+'us'='ARDUOUS'
(Other definitions for arduous that I've seen before include "Daunting: strenuous" , "Steep; laborious" , "Difficult, laborious" , "Requiring hard work" , "Difficult, tiring" .)