Gossip who lies about lake monster back in high land (15)
I believe the answer is:
flibbertigibbet
'gossip' is the definition.
(thesaurus)
'who lies about lake monster back in high land' is the wordplay.
I cannot really understand how this works, but
'lake' could be 'l' and 'l' is found within the answer.
'back' could be 'bet' and 'bet' is located in the answer.
'who lies' could be 'fibber' and 'fibber' is present in the leftover letters.
This explanation may well be incorrect...
(Other definitions for flibbertigibbet that I've seen before include "Frivolous or flighty person" , "Till Eulenspiegel, say" , "Gossip" , "Flighty type" , "One who's unreliable" .)