Legendary little guy developing Herculean power is inspired (10)
I believe the answer is:
leprechaun
'legendary little guy' is the definition.
(imaginary little creature)
'developing herculean power is inspired' is the wordplay.
'developing' indicates anagramming the letters (letters developed into a new form).
'power' becomes 'p' (symbol in physics).
'is inspired' indicates putting letters inside (inserted letters are breathed in or inspired).
'herculean' with letters rearranged gives 'lerechaun'.
'lerechaun' enclosing 'p' is 'LEPRECHAUN'.
(Other definitions for leprechaun that I've seen before include "Mischievous Irish elf" , "Mischievous imp" , "He helps Irish housewife" , "Mischievous Irish sprite" , "Elf in Irish folklore" .)