Again our capital territory initially seriously responds (6)
I believe the answer is:
reacts
'responds' is the definition.
('react' can be a synonym of 'respond')
'again our capital territory initially seriously' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'our' becomes 'ac' (I can't explain this - if you can you should believe this answer much more).
'capital' says to take the initial letters.
'initially' suggests taking the first letters.
The first letter of 'territory' is 't'.
The first letter of 'seriously' is 's'.
're'+'ac'+'t'+'s'='REACTS'
Can you help me to learn more?
(Other definitions for reacts that I've seen before include "Responds to an impulse" , "Retaliates" , "Responds to stimulus (6)" , "Acts in response to something" .)