Hiding in corner, youngster, stubborn (6)
I believe the answer is:
ornery
'stubborn' is the definition.
(I've seen this in another clue)
'hiding in corner youngster' is the wordplay.
'hiding' indicates the answer is hidden within the clue.
'ORNERY' can be found hidden inside 'in corner youngster'.
(Other definitions for ornery that I've seen before include "Cantankerous or mean (US)" , "Mean - stubborn" , "Difficult to deal with" , "Awkward" .)