To mistake them for lemons would be silly (6)
I believe the answer is:
melons
'to mistake them' is the definition.
I can't judge whether this defines the answer.
'lemons would be silly' is the wordplay.
'lemons' can be anagrammed to 'MELONS'.
Though, I am not sure how the anagram is indicated.
'for' is the link.
Can you help me to learn more?
(Other definitions for melons that I've seen before include "Fruits with hard rinds" , "Juicy fruits" , "Lemons turn out to be other fruits" , "Lemons make other fruits" , "Juicy fruit" .)