Mediocre song by cat, messing about (4-2-8)

'mediocre' is the definition.
(thesaurus)

'song by cat messing about' is the wordplay.
I cannot really understand how this works, but
'song' could be 'air' (air can mean a piece of music) and 'air' is located in the answer.
'cat' could be 'tom' (a tom is a male cat) and 'tom' is found within the answer.
The remaining letters 'fiddling' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?