During sunny day, scoff that lady's underwear is silly (14)

'silly' is the definition.
(synonyms)

'during sunny day scoff that lady's underwear' is the wordplay.
I cannot quite see how this works, but
'during' could be 'in' (e.g. during February means in February) and 'in' is found in the answer.
'day' could be 'd' (abbreviation for day) and 'd' is found in the answer.
'scoff' could be 'eat' (to scoff something is to eat it quickly) and 'eat' is found in the answer.
'lady's' could be 'her' (belonging to a lady) and 'her' is located in the answer.
'underwear' could be 'bra' (bra is a type of undergarment) and 'bra' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'is' acts as a link.

Can you help me to learn more?