Reaction of press with one running riot (8)
I believe the answer is:
response
'reaction' is the definition.
(synonyms)
'press with one running riot' is the wordplay.
'with' means one lot of letters go next to another.
'running riot' indicates anagramming the letters.
'press' put after 'one' is 'onepress'.
'onepress' anagrammed gives 'RESPONSE'.
'of' acts as a link.
(Other definitions for response that I've seen before include "Reply, rejoinder" , "Preens so by way of answer" , "'Answer, reply (8)'" , "Reaction, reply" , "bid by partner" .)