Sound Queen turned up when single should end (8)
I believe the answer is:
reasoned
'sound' is the definition.
('reasoned' can be a synonym of 'sound')
'queen turned up when single should end' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina**).
'turned up' shows that the letters should be reversed in order.
'when' becomes 'as'.
'single' becomes 'one' (I've seen this before).
'end' suggests the final letters.
The final letter of 'should' is 'd'.
'er' back-to-front is 're'.
're'+'as'+'one'+'d'='REASONED'
(Other definitions for reasoned that I've seen before include "Logically valid" , "Dear ones (anag)" , "Argued logically" , "Well thought out" , "Based on logic" .)