Reacted by putting marsh grass round the south pond (9)
I believe the answer is:
responded
'reacted by' is the definition.
'responded' can be an answer for 'reacted' (responding is a kind of reacting). I am not certain of the 'by' bit.
'marsh grass round the south pond' is the wordplay.
'marsh grass' becomes 'reed' (I've seen this in another clue).
'round' indicates putting letters inside.
'the south' becomes 's' (synonyms).
's'+'pond'='spond'
'reed' enclosing 'spond' is 'RESPONDED'.
'putting' acts as a link.
Can you help me to learn more?
(Other definitions for responded that I've seen before include "Came back" , "Answered or reacted" , "Replied, reacted" .)