The lads seem, to you, thus non-U! (6)
I believe the answer is:
youths
'the lads' is the definition.
(I've seen this in another clue)
'you thus non-u' is the wordplay.
'non' is a deletion indicator.
'thus' with 'u' taken away is 'ths'.
'you'+'ths'='YOUTHS'
'seem to' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for youths that I've seen before include "Males between childhood and maturity" , "Adolescents" , "Youngsters" , "juveniles" , "kids" .)