Ring round vessel during the day (7)
I believe the answer is:
diurnal
'during the day' is the definition.
(I've seen this before)
'ring round vessel' is the wordplay.
'ring' becomes 'dial' (I've seen this before).
'round' means one lot of letters goes inside another.
'vessel' becomes 'urn' (urn is a kind of vessel).
'dial' enclosing 'urn' is 'DIURNAL'.
(Other definitions for diurnal that I've seen before include "Active during the day" , "Of the day, not the night" , "Happening daily" , "Daily; during the daytime" , "I run lad in daily mode, not nocturnal" .)