Clue: Literary heroine, awfully gung-ho on the inside, but lacking good resilience (9)
Answer: TOUGHNESS
Explanation: Definition: resilience

T**ESS** (of the D'Urbervilles, a "literary heroine") with ***(ungho)** [anag:*awfully*] *on the inside*, where UNGHO is (g)UNG-HO lacking G (good)