Reasonable tune queen's mate used in playing violin (4,2,8)
I believe the answer is:
fair to middling
'reasonable' is the definition.
(phrase meaning reasonable or of average quality)
'tune queen's mate used in playing violin' is the wordplay.
'tune' becomes 'air' (synonyms).
'queen's mate' becomes 'tom' (a queen is a female cat).
'used in' indicates putting letters inside.
'playing violin' becomes 'fiddling' (fiddle can mean a violin).
'air'+'tom'='airtom'
'airtom' going into 'fiddling' is 'FAIR TO MIDDLING'.
(Other definitions for fair to middling that I've seen before include "Unremarkable" , "Not bad" , "OK?" , "unexceptional" , "average" .)