Destroying Hydra around lake only with great difficulty (6)
I believe the answer is:
hardly
'only with great difficulty' is the definition.
(done only with great difficulty)
'destroying hydra around lake' is the wordplay.
'destroying' indicates anagramming the letters.
'around' means one lot of letters goes inside another.
'lake' becomes 'l' (geographical abbreviation).
'hydra' with letters rearranged gives 'hardy'.
'hardy' going around 'l' is 'HARDLY'.
(Other definitions for hardly that I've seen before include "Almost not" , "Barely, scarcely" , "Not much" , "With great difficulty" , "Not quite "It seems unlikely!"" .)