Little girl, as one doing nothing in a brilliant way (7)
I believe the answer is:
vividly
'in a brilliant way' is the definition.
'little girl as one doing nothing' is the wordplay.
'little girl' becomes 'viv' (short for Vivienne).
'as one doing nothing' becomes 'idly' (like an idle person).
'viv'+'idly'='VIVIDLY'
(Other definitions for vividly that I've seen before include "vigorous manner" , "Brightly, vibrantly" .)