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 upset by' indicates anagramming the letters.
'some silly lark' becomes 'ral' (I am not sure about this - if you are sure you should believe this answer much more).
'aunt' anagrammed gives 'natu'.
'natu'+'ral'='NATURAL'
'for' acts as a 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" .)