It's silly to be very noisy during the day (5)
I believe the answer is:
daffy
'it's silly' is the definition.
(I've seen this before)
'very noisy during the day' is the wordplay.
'very noisy' becomes 'ff'.
'during' is an insertion indicator.
'ff' put within 'day' is 'DAFFY'.
'to be' acts as a link.
(Other definitions for daffy that I've seen before include "Silly, mildly eccentric" , "Crazy" , "duck on screen" , "Bonkers" , "Silly, eccentric (colloq.)" .)