Big lake with flat-bottomed boat heading off (5)
I believe the answer is:
large
'big' is the definition.
(synonyms)
'lake with flat-bottomed boat heading off' is the wordplay.
'lake' becomes 'l' (geographical abbreviation).
'with' is a charade indicator (letters next to each other).
'flat-bottomed boat' becomes 'barge' (I've seen this before).
'heading off' means to remove the first letter.
'barge' with its initial letter removed is 'arge'.
'l'+'arge'='LARGE'
(Other definitions for large that I've seen before include "Generous" , "Vast" , "substantial" , "Ample" , "Great being regal" .)