About to perform and respond (5)
I believe the answer is:
react
'respond' is the definition.
(to react to something is to respond to it)
'about to perform' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'to perform' becomes 'act' (acting is a kind of performing).
're'+'act'='REACT'
'and' acts as a link.
(Other definitions for react that I've seen before include "Change chemically" , "What a chemical might do" , "Respond to something" , "Perform again" , "Show opposition" .)