Lines packed into day’s material (5)
I believe the answer is:
tulle
'material' is the definition.
(tulle is a kind of material)
'lines packed into day's' is the wordplay.
'lines' becomes 'l' (abbreviation as in lines of poetry).
'packed' becomes 'l' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'into' means one lot of letters goes inside another.
'day' becomes 'tue' (abbreviation for Tuesday).
'l'+'l'='ll'
'll' inserted within 'tue' is 'TULLE'.
Can you help me to learn more?
(Other definitions for tulle that I've seen before include "Thin material" , "Fine net fabric used for hat trimming, etc." , "Fine silky net used in veils" , "Tutu netting" , "Delicate netted fabric" .)