Mischievous wife rubbished his claim (9)
I believe the answer is:
whimsical
'mischievous' is the definition.
(I've seen this before)
'wife rubbished his claim' is the wordplay.
'wife' becomes 'w' (abbreviation).
'rubbished' indicates anagramming the letters.
'his'+'claim'='hisclaim'
'hisclaim' anagrammed gives 'himsical'.
'w'+'himsical'='WHIMSICAL'
(Other definitions for whimsical that I've seen before include "Capricious" , "Fanciful or playful" , "Appealingly fanciful" , "Playfully quaint" , "Quaintly fantastic" .)