In on-air broadcast, deejay's opening line is so-so (8)
I believe the answer is:
ordinary
'is so-so' is the definition.
(similar in meaning)
'in on-air broadcast deejay's opening line' is the wordplay.
'in' indicates putting letters inside.
'broadcast' is an anagram indicator (letters scattered or cast around).
'opening' indicates taking the first letters.
'line' becomes 'ry' (abbreviation for railway).
The first letter of 'deejays' is 'd'.
'on'+'air'='onair'
'onair' anagrammed gives 'orina'.
'orina' enclosing 'd' is 'ordina'.
'ordina'+'ry'='ORDINARY'
(Other definitions for ordinary that I've seen before include "simple charge" , "'Commonplace, banal (8)'" , "Usual - bishop in his province" , "Normal, usual" , "Run of the mill" .)