That is to cover the girl in, silly! (5)
I believe the answer is:
inane
'silly' is the definition.
(inane means silly or foolish)
'that is to cover the girl' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'to cover' indicates putting letters inside.
'the girl' becomes 'nan' (Nan can be short for Nancy or Anne).
'ie' enclosing 'nan' is 'INANE'.
'in' acts as a link.
(Other definitions for inane that I've seen before include "Thick" , "Empty" , "Stupid and silly" , "Daft" , "Empty-headed" .)