Be exultant regarding Irish writer on the radio (7)
I believe the answer is:
rejoice
'be exultant' is the definition.
(I've seen this before)
'regarding irish writer on the radio' is the wordplay.
'regarding' becomes 're' ('re' means regarding or about).
'irish writer' becomes 'joyce' (author James Joyce).
'on the radio' indicates a 'sounds like' (homophone) clue.
'joyce' sounds like 'joice'.
're'+'joice'='REJOICE'
(Other definitions for rejoice that I've seen before include "Feel great delight" , "Be happy" , "Show great delight" , "Express great happiness" , "Exult, show great joy" .)