Light as a soft drink (4)
I believe the answer is:
pale
'light' is the definition.
(similar in meaning)
'a soft drink' is the wordplay.
'a soft' becomes 'p' (musical abbreviation for piano, 'soft').
'drink' becomes 'ale' (type of beer).
'p'+'ale'='PALE'
'as' is the link.
(Other definitions for pale that I've seen before include "Wan; boundary" , "stick" , "A bit of fencing" , "Upright member" , "Without much colour" .)