Again pretend to respond (5)
I believe the answer is:
react
'to respond' is the definition.
(to react to something is to respond to it)
'again pretend' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'pretend' becomes 'act' (I've seen this in another clue).
're'+'act'='REACT'
(Other definitions for react that I've seen before include "Hit back" , "Show opposition" , "Change chemically" , "Behave in response" , "Trace (anag)" .)