Strength of a Hardy heroine from the south (5)
I believe the answer is:
asset
'strength' is the definition.
(I've seen this before)
'a hardy heroine from the south' is the wordplay.
'hardy heroine' becomes 'tess' (in Tess of the d'Urbervilles, Thomas Hardy novel).
'from the south' is a reversal indicator (in a down clue, letters go up).
'tess' reversed gives 'sset'.
'a'+'sset'='ASSET'
'of' is the link.
(Other definitions for asset that I've seen before include "Useful or valuable quality or possession" , "Something well worth having" , "Useful item" , "Something valuable to have" , "Beneficial thing" .)