Past letters call for mercy (6)
I believe the answer is:
pathos
'for mercy' is the definition.
(thesaurus)
'past letters call' is the wordplay.
I cannot really see how this works, but
'call' could be 'ho' (historical exclamation used to attract attention) and 'ho' is found in the answer.
an anagram of 'past' is 'pats' which is present in the left over letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for pathos that I've seen before include "The quality moves one to pity" , "Quality that arouses sympathy" , "Quality evoking pity or sadness" , "Something evoking pity" , "At shop for poignancy" .)