Hydra moving across lake only with great difficulty (6)
I believe the answer is:
hardly
'only with great difficulty' is the definition.
(done only with great difficulty)
'hydra moving across lake' is the wordplay.
'moving' indicates anagramming the letters.
'across' is an insertion indicator.
'lake' becomes 'L' (geographical abbreviation).
'hydra' with letters rearranged gives 'hardy'.
'hardy' enclosing 'l' is 'HARDLY'.
(Other definitions for hardly that I've seen before include "Not much" , "Not quite "It seems unlikely!"" , "Barely, scarcely" , "With great difficulty" , "Almost not" .)