Fizzy drink tons mostly kept ready (5)
I believe the answer is:
tonic
'fizzy drink' is the definition.
'tonic' can be an answer for 'drink' (tonic is a kind of drink). I am not sure about the 'fizzy' bit.
'tons mostly kept ready' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'kept ready' becomes 'ic' (I can't justify this - if you can you should give a lot more credence to this answer).
'tons' with its last letter taken away is 'ton'.
'ton'+'ic'='TONIC'
Can you help me to learn more?
(Other definitions for tonic that I've seen before include "something providing a boost" , "Restorative that invigorates" , "Stimulant" , "Quinine drink; keynote" , "Accompaniment to gin?" .)