Little adolescent is extremely sarky (6)
I believe the answer is:
teensy
'little' is the definition.
(similar in meaning)
'adolescent is extremely sarky' is the wordplay.
'adolescent' becomes 'teen' (teenager).
'is extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'sarky' with its middle removed is 'sy'.
'teen'+'sy'='TEENSY'
(Other definitions for teensy that I've seen before include "Minute" , "- - -weensy" , "Tiny" , "Very small" , "Mini" .)