Nut, alas, mistaken for fruit (7)
I believe the answer is:
sultana
'fruit' is the definition.
(sultana is a kind of fruit)
'nut alas mistaken' is the wordplay.
'mistaken' indicates an anagram.
'nut'+'alas'='nutalas'
'nutalas' is an anagram of 'SULTANA'.
'for' acts as a link.
(Other definitions for sultana that I've seen before include "Sovereign's mother perhaps" , "Muslim ruler's wife perhaps" , "Sovereign's mistress" , "Dry seedless grape" , "pudding ingredient" .)