Mathematical item: arts graduate gets confused on the radio (6)
I believe the answer is:
matrix
'mathematical item' is the definition.
(display of numbers in mathematics)
'arts graduate gets confused on the radio' is the wordplay.
'arts graduate' becomes 'ma' (Master of Arts).
'gets confused' becomes 'tricks'.
'on the radio' shows a homophone (sound like).
'tricks' is a homophone of 'trix'.
'ma'+'trix'='MATRIX'
(Other definitions for matrix that I've seen before include "Mathematical array; The --, sci-fi film" , "Mould for, eg, record" , "Grid-like array of elements" , "network of circuit elements" , "Grid-like pattern of elements" .)