Thus, half days can be refreshing (4)
I believe the answer is:
soda
'be refreshing' is the definition.
I can't tell whether this definition defines the answer.
'thus half days' is the wordplay.
'thus' becomes 'so' (synonyms).
'half' indicates taking half.
'days' cut in half is 'da'.
'so'+'da'='SODA'
'can' is the link.
Can you help me to learn more?
(Other definitions for soda that I've seen before include "Fizzy water, goes well with whiskey" , "Whisky mixer" , "pressurised liquid" , "Accompaniment to whisky" , "In the US, a sweet fizzy drink" .)