Flower covering half of lake? That's probable (6)
I believe the answer is:
likely
'probable' is the definition.
(I know that probable can be written as likely)
'flower covering half of lake?' is the wordplay.
'flower' becomes 'lily' (I've seen this before).
'covering' means one lot of letters goes inside another.
'half of' indicates taking half.
'lake' cut in half is 'ke'.
'lily' enclosing 'ke' is 'LIKELY'.
'that's' acts as a link.
(Other definitions for likely that I've seen before include "Promising, probably" , "Ilkley (anag.)" , "In all probability" , "In prospect" , "Probable, expected" .)