The Queen repelled by a relative's motive (6)
I believe the answer is:
reason
'motive' is the definition.
(I know that reason is a type of rational motive)
'the queen repelled by a relative's' is the wordplay.
'the queen' becomes 'ER' (Elizabeth Regina).
'repelled' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'relative' becomes 'son' (son is a kind of relative).
'er' back-to-front is 're'.
're'+'a'+'son'='REASON'
(Other definitions for reason that I've seen before include "brain operations" , "Cause - logic" , "Mental power to justify an act" , "Power of thought" , "Sanity, the ability to think" .)