What's sweet in big book, lines offering nonsense (10)
I believe the answer is:
tomfoolery
'nonsense' is the definition.
(thesaurus)
'what's sweet in big book lines' is the wordplay.
'what's sweet' becomes 'fool' (type of dessert or sweet).
'in' indicates putting letters inside.
'big book' becomes 'tome' (I've seen this before).
'lines' becomes 'ry' (abbreviation for railway).
'fool' going within 'tome' is 'tomfoole'.
'tomfoole'+'ry'='TOMFOOLERY'
'offering' is the link.
(Other definitions for tomfoolery that I've seen before include "Craziness" , "Nonsense" , "hanky-panky" , "Shenanigans" , "Acting stupidly" .)