Impossible if blues fan changes, accepting Queen (12)
I believe the answer is:
insufferable
'impossible' is the definition.
('insufferable' can be a synonym of 'impossible')
'if blues fan changes accepting queen' is the wordplay.
'changes' indicates an anagram.
'accepting' indicates putting letters inside.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'if'+'blues'+'fan'='ifbluesfan'
'ifbluesfan' is an anagram of 'insuffable'.
'insuffable' enclosing 'er' is 'INSUFFERABLE'.
(Other definitions for insufferable that I've seen before include "Too much to bear" , "Unbearably conceited" , "Unbearably arrogant" .)