Expertise on the radio? Not at all (5)
I believe the answer is:
nohow
'not at all' is the definition.
(synonyms)
'expertise on the radio?' is the wordplay.
'expertise' becomes 'know-how' (expertise in a particular area).
'on the radio?' indicates a 'sounds like' (homophone) clue (I've seen 'on the radio' mean this).
'knowhow' is a homophone of 'NOHOW'.
(Other definitions for nohow that I've seen before include "Not in any way (colloq.)" , "Definitely not" , "Not by any means, informally" , "not at all the American way?" , "Never" .)