"The quality of ... is not strained" (Merchant of Venice) (5)
I believe the answer is:
mercy
(Other definitions for mercy that I've seen before include "Shakespeare 'The quality of . . . . . is not strained'" , "'The quality of ... is not strained' (The Merchant of Venice)" , "Clemency or compassion" , "Compassion for the unfortunate" , "Its quality is not strained, Portia said" .)