Vast lake on isle obscured with mist (9)
I believe the answer is:
limitless
'vast' is the definition.
(synonyms)
'lake on isle obscured with mist' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'on' is an insertion indicator.
'obscured with' indicates anagramming the letters (I've seen 'obscure' mean this).
'isle' mixed up with 'mist' makes 'limitess'.
'l' put inside 'limitess' is 'LIMITLESS'.
(Other definitions for limitless that I've seen before include "Very large, extensive" , "Without bounds" , "immeasurable" , "no end" , "All-pervasive" .)