Boasted about day being jam-packed (7)
I believe the answer is:
crowded
'jam-packed' is the definition.
(similar in meaning)
'boasted about day' is the wordplay.
'boasted' becomes 'crowed' (crowing is a kind of boasting).
'about' indicates putting letters inside.
'day' becomes 'd' (abbreviation for day).
'crowed' going around 'd' is 'CROWDED'.
'being' acts as a link.
(Other definitions for crowded that I've seen before include "cramped" , "Packed (of people)" , "Jam-packed" , "With many present" , "With too many people" .)