Tree in hideaway by lake (6)
I believe the answer is:
linden
'tree' is the definition.
(I know that linden is a type of tree)
'in hideaway by lake' is the wordplay.
'hideaway' becomes 'den' (synonyms).
'by' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
'in'+'den'='inden'
'inden' after 'l' is 'LINDEN'.
(Other definitions for linden that I've seen before include "Lime tree" , "Tree with Lea - also called 8" , "type of tree" , "One twigged" , "Unter den ------, iconic boulevard in Berlin" .)