One day out of drink so reserved more? (5)
I believe the answer is:
icier
'of drink so reserved more?' is the definition.
I can't judge whether this defines the answer.
'one day out' is the wordplay.
'one' becomes 'ici' (I can't explain this - if you can you should give a lot more credence to this answer).
'day' becomes 'era' (day can mean an era of time).
'out' means to remove the last letter.
'era' with its last letter taken away is 'er'.
'ici'+'er'='ICIER'
Can you help me to learn more?
(Other definitions for icier that I've seen before include "More cold or frozen" , "More reserved" , "More glacial" , "More slippery" , "With more hostility" .)