Irritable youth, initially under examination (5)
I believe the answer is:
testy
'irritable' is the definition.
('testy' can be a synonym of 'irritable')
'youth initially under examination' is the wordplay.
'youth initially' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'under' says to put letters next to each other.
'examination' becomes 'test' ('test' can be a synonym of 'examination').
'y' put after 'test' is 'TESTY'.
Can you help me to learn more?
(Other definitions for testy that I've seen before include "Captious" , "Short-tempered" , "Irritably impatient" , "snappy" , "Grumpy" .)