Young Sidney put back inside; very, very silly (6)
I believe the answer is:
'very silly' is the definition.
(I've seen this before)
'young sidney put back inside' is the wordplay.
'young sidney' becomes 'sid' (Sid is short for Sidney. I am not sure about the 'young' bit.).
'back' says the letters should be written in reverse.
'inside' indicates putting letters inside.
'put' reversed gives 'tup'.
'sid' placed around 'tup' is 'STUPID'.
'very' acts as a link.
Can you help me to learn more?
(Other definitions for stupid that I've seen before include "Senseless" , "Very silly" , "bovine" , "Brainless" , "Irresponsible" .)