Feeling tense in front of teacher being nasty (8)
I believe the answer is:
sentient
'feeling' is the definition.
('sentient' can be a synonym of 'feeling')
'tense in front of teacher being nasty' is the wordplay.
'front of' says to take the initial letters.
'being nasty' indicates an anagram.
The first letter of 'teacher' is 't'.
'tense'+'in'+'t'='tenseint'
'tenseint' is an anagram of 'SENTIENT'.
(Other definitions for sentient that I've seen before include "Endowed with feeling and consciousness" , "Having feelings" , "Able to perceive/feel" , "Conscious - aware" , "Capable of feeling things as in tenets" .)