Great river in time getting to lake (5)
I believe the answer is:
large
'great' is the definition.
('large' can be similar in meaning to 'great')
'river in time getting to lake' is the wordplay.
'river' becomes 'r'.
'in' indicates putting letters inside.
'time' becomes 'age' (age is a kind of time**).
'getting' means one lot of letters go next to another.
'to lake' becomes 'l'.
'r' going into 'age' is 'arge'.
'arge' after 'l' is 'LARGE'.
(Other definitions for large that I've seen before include "Sizeable" , "Big, almost regal" , "Great being regal" , "Strapping" , "Wide-ranging" .)