Youngster put to bed and left alone (9)
I believe the answer is:
boycotted
'left alone' is the definition.
The answer and definition can be both to do with social activities as well as being past participle verbs.
Perhaps they are linked in a way I don't understand?
'youngster put to bed' is the wordplay.
'youngster' becomes 'boy' (I've seen this in another clue).
'put to bed' becomes 'cotted' (I can't justify this - if you can you should give a lot more credence to this answer).
'boy'+'cotted'='BOYCOTTED'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for boycotted that I've seen before include "Shunned (something) in protest" , "Ostracised by octet body" .)