'Sunny' lake spotted in upward journey (5)
I believe the answer is:
solar
'sunny' is the definition.
(I've seen this in another clue)
'lake spotted in upward journey' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'spotted in' is an insertion indicator.
'upward journey' becomes 'soar' (soaring upwards).
'l' inserted into 'soar' is 'SOLAR'.
(Other definitions for solar that I've seen before include "of heavenly body" , "Relating to the sun" , "revolutionary system" , "Using the energy of the sun" , "room" .)