What can refresh one’s day, coming in mostly sluggish (8)
I believe the answer is:
lemonade
'what can refresh one's' is the definition.
(drink that can refresh you)
'day coming in mostly sluggish' is the wordplay.
'day' becomes 'mon' (abbreviation for Monday).
'coming in' indicates putting letters inside.
'mostly' means to remove the last letter.
'sluggish' becomes 'leaden'.
'leaden' with its last letter taken off is 'leade'.
'mon' inserted inside 'leade' is 'LEMONADE'.
(Other definitions for lemonade that I've seen before include "Sweet carbonated drink" , "Ale demon (anag)" , "(Usually) fizzy drink" , "Still or fizzy drink" , "Non-alcoholic drink" .)