Tough as anything in heartless duty (7)
I believe the answer is:
doughty
'tough' is the definition.
(thesaurus)
'anything in heartless duty' is the wordplay.
'anything' becomes 'ought' (dialect term for 'anything').
'in' indicates putting letters inside.
'heartless' says to hollow out the word (remove centre letters).
'duty' with its centre taken out is 'dy'.
'ought' placed within 'dy' is 'DOUGHTY'.
'as' acts as a link.
(Other definitions for doughty that I've seen before include "Stout-hearted, valiant" , "Brave, resolute (arch.)" , "Fearless" .)