Very small adolescent, extremely sulky (6)
I believe the answer is:
teensy
'very small' is the definition.
(I've seen this in another clue)
'adolescent extremely sulky' is the wordplay.
'adolescent' becomes 'teen' (teenager).
'extremely' suggests removing the centre (only the extremes of the word are used).
'sulky' with its centre removed is 'sy'.
'teen'+'sy'='TEENSY'
(Other definitions for teensy that I've seen before include "- - -weensy" , "Tiny" , "Minute" , "Mini" , "Very small" .)