Folly of girl crossing North America (6)
I believe the answer is:
lunacy
'folly' is the definition.
(thesaurus)
'girl crossing north america' is the wordplay.
'girl' becomes 'lucy' (**).
'crossing' is an insertion indicator.
'north america' becomes 'na'.
'lucy' enclosing 'na' is 'LUNACY'.
'of' is the link.
(Other definitions for lunacy that I've seen before include "Madness, insanity" , "Tomfoolery" , "Foolishness" , "Craziness, madness" , "Moon madness" .)