Busy individual penning lines for stunner (5)
I believe the answer is:
belle
'stunner' is the definition.
(thesaurus)
'busy individual penning lines' is the wordplay.
'busy individual' becomes 'bee' (I can't explain this - if you can you should believe this answer much more).
'penning' means one lot of letters goes inside another.
'lines' becomes 'll' (two abbreviations for line).
'bee' going around 'll' is 'BELLE'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for belle that I've seen before include "Reigning beauty" , "-- Epoque, before the Great War" , "Beautiful woman - of the ball?" , "Most attractive girl" , "dish" .)