Magical words of Aesop seem silly about lion's tail (4,6)
I believe the answer is:
open sesame
'magical words' is the definition.
(I've seen this before)
'aesop seem silly about lion's tail' is the wordplay.
'silly' indicates an anagram.
'about' means one lot of letters goes inside another.
'tail' says to take the final letters.
The last letter of 'lion' is 'n'.
'aesop'+'seem'='aesopseem'
'aesopseem' anagrammed gives 'opesesame'.
'opesesame' placed around 'n' is 'OPEN SESAME'.
'of' acts as a link.
(Other definitions for open sesame that I've seen before include "Spell unlocking doors" , "free entry" , "Spell from Ali Baba and the Forty Thieves" , "door-unlocking spell" , "Words of Ali Baba" .)