Resigned from Saint Osyth's -- originally in charge (5)
I believe the answer is:
stoic
'resigned' is the definition.
(I've seen this before)
'saint osyth's originally in charge' is the wordplay.
'saint' becomes 'st'.
'originally' suggests taking the first letters.
'in charge' becomes 'ic'.
The initial letter of 'osyth' is 'o'.
'st'+'o'+'ic'='STOIC'
'from' is the link.
(Other definitions for stoic that I've seen before include "One patient under misfortune" , "Greek philosopher" , "Impassive type" , "philosophical type" , "Solemn ceremony" .)