Lax concerning young lady (6)
I believe the answer is:
remiss
'lax' is the definition.
(I've seen this before)
'concerning young lady' is the wordplay.
'concerning' becomes 're'.
'young lady' becomes 'miss' ('miss' can be a synonym of 'young lady'**).
're'+'miss'='REMISS'
(Other definitions for remiss that I've seen before include "Failing in one's duty" , "Negligent, slack" , "Wrong" , "Derelict in duty" , "Lacking care to duty" .)