That is to cover the girl in, silly! (5)
I believe the answer is:
inane
'silly' is the definition.
(thesaurus)
'that is to cover the girl' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'to cover' means one lot of letters goes inside another.
'the girl' becomes 'nan' (**).
'ie' going around 'nan' is 'INANE'.
'in' is the link.
(Other definitions for inane that I've seen before include "Thick" , "Empty" , "Stupid and silly" , "Daft" , "Empty-headed" .)