Almost everyone beset by vermin will show hostility (6)
I believe the answer is:
malice
'hostility' is the definition.
(thesaurus)
'almost everyone beset by vermin' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'everyone' becomes 'all' (all people).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'vermin' becomes 'mice' ().
'all' with its last letter removed is 'al'.
'al' inserted inside 'mice' is 'MALICE'.
'will show' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for malice that I've seen before include "Bitterness" , "Spite - desire to hurt" , "Spiteful, animosity, sometimes a forethought" , "Ill will, a desire to harm others" , "Spite and rancour" .)