Firm, beset by safe crackers, not hearing a thing (5-4)
I believe the answer is:
stone-deaf
'not hearing a thing' is the definition.
I can't tell whether this defines the answer.
'firm beset by safe crackers' is the wordplay.
'firm' becomes 'toned' (I've seen this before).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'crackers' is an anagram indicator (informal term for mad).
'safe' with letters rearranged gives 'seaf'.
'toned' going inside 'seaf' is 'STONE-DEAF'.
Can you help me to learn more?
(Other definitions for stone-deaf that I've seen before include "No defeats, being completely without hearing" , "Completely unable to hear" , "Tones fade, being quite without hearing" , "Feasted on (anag.)" , "Totally unable to hear" .)