Shame troops seizing southern capital in rebellion (7)
I believe the answer is:
remorse
'shame' is the definition.
(thesaurus)
'troops seizing southern capital in rebellion' is the wordplay.
'troops' becomes 'RE' (Royal Engineers army regiment).
'seizing' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'capital in rebellion' becomes 'more' (I can't explain this - if you can you should give a lot more credence to this answer).
's' placed inside 'more' is 'morse'.
're'+'morse'='REMORSE'
(Other definitions for remorse that I've seen before include "regrets" , "Pity" , "Sorrow for wrongs committed" , "Regret for wrong" , "Feeling of guilt" .)