Small youngster's last day (6)
I believe the answer is:
teensy
'small' is the definition.
(similar in meaning)
'youngster's last day' is the wordplay.
'youngster's' becomes 'teen's' (teenager).
'last' suggests the final letters.
The final letter of 'day' is 'y'.
'teens'+'y'='TEENSY'
(Other definitions for teensy that I've seen before include "- - -weensy" , "Very small" , "Mini" , "Minute" , "Tiny" .)