Likely to respond and foolishly create about X's opposite number (8)
I believe the answer is:
reactive
'likely to respond' is the definition.
'foolishly create about x's opposite number' is the wordplay.
'foolishly' is an anagram indicator.
'about' indicates putting letters inside.
'x's opposite number' becomes 'iv' (opposite number to X on a Roman numerals clock).
'create' is an anagram of 'reacte'.
'reacte' placed around 'iv' is 'REACTIVE'.
'and' acts as a link.
(Other definitions for reactive that I've seen before include "Responding to stimulus" , "Producing responses" , "Responding to stimuli" , "Showing a response to stimuli" , "Responsive to stimulus" .)