Harried administrator rejects torrid letters, demonstrating resilience (7)
I believe the answer is:
stamina
'resilience' is the definition.
(I've seen this before)
'harried administrator rejects torrid' is the wordplay.
'harried' is an anagram indicator.
'rejects' indicates named letters should be taken away.
'administrator' with 'torrid' taken out is 'amnisat'.
'amnisat' with letters rearranged gives 'STAMINA'.
'letters demonstrating' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for stamina that I've seen before include "capability to keep going" , "Resilience" , "Ability for sustained effort" , "Sustained energy" , "The ability to sustain prolonged physical or mental effort" .)