I'm upset by the strain of being a teacher (8)
I believe the answer is:
mistress
'a teacher' is the definition.
(I know that mistress is a type of school teacher)
'i'm upset by the strain' is the wordplay.
'upset by' is an anagram indicator.
'the strain' becomes 'stress' ('stress' can be a synonym of 'strain').
'im' is an anagram of 'mi'.
'mi'+'stress'='MISTRESS'
'of being' acts as a link.
(Other definitions for mistress that I've seen before include "Schoolteacher" , "Female teacher" , "Woman in control" , "Kept woman" , "Husband's paramour" .)