Responded concerning new cadet (7)
I believe the answer is:
reacted
'responded' is the definition.
('react' can be a synonym of 'respond')
'concerning new cadet' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'new' indicates anagramming the letters.
'cadet' is an anagram of 'acted'.
're'+'acted'='REACTED'
(Other definitions for reacted that I've seen before include "Showed a response" , "Responded to a stimulus" .)