It's reasonable for aunt to be upset by some silly lark! (7)
I believe the answer is:
natural
'it's reasonable' is the definition.
(thesaurus)
'aunt to be upset by some silly lark' is the wordplay.
'to be' indicates anagramming the letters.
'upset' shows that the letters should be reversed in order.
'by some silly lark' becomes 'lar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lar' back-to-front is 'ral'.
'aunt' anagrammed gives 'natu'.
'natu'+'ral'='NATURAL'
'for' is the link.
Can you help me to learn more?
(Other definitions for natural that I've seen before include "Inborn; unaffected" , "without man's involvement" , "character on the staff" , "Uncultivated" , "Unspoilt" .)