The formidable hard-hearted pair before us (7)
I believe the answer is:
arduous
'the formidable' is the definition.
(thesaurus)
'hard-hearted pair before us' is the wordplay.
'hearted' means to look at the middle letters.
'pair' becomes 'duo' ('duo' can be a synonym of 'pair'**).
'before' says to put letters next to each other.
The centre of 'hard' is 'ar'.
'ar'+'duo'+'us'='ARDUOUS'
(Other definitions for arduous that I've seen before include "Demanding great effort" , "Punishing" , "Extremely taxing" , "Requiring much effort" , "Difficult, tiring" .)