He, having left the county, teaches school! (3)
I believe the answer is:
sir
'he having left the county teaches school' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he having left the county' is the wordplay.
'having left' indicates named letters should be taken away.
'the county' becomes 'shire' (term for a county, eg Hampshire, Warwickshire etc).
'shire' with 'he' removed is 'SIR'.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Polite term of address for a man" , "Gentleman's form of address" , "Feted fellow" , "Formal and deferential address to a man" , "... ___ John Major" .)