Delinquent young lady chasing soldiers (6)
I believe the answer is:
remiss
'delinquent' is the definition.
('remiss' can be a synonym of 'delinquent')
'young lady chasing soldiers' is the wordplay.
'young lady' becomes 'miss' ('miss' can be a synonym of 'young lady'**).
'chasing' means one lot of letters go next to another.
'soldiers' becomes 're' (Royal Engineers).
'miss' after 're' is 'REMISS'.
(Other definitions for remiss that I've seen before include "Failing in one's duty" , "Negligent, lax" , "Careless concerning a girl?" , "delinquent" , "Failing in what duty requires" .)