Pen used round old lines in artwork (7)
I believe the answer is:
collage
'artwork' is the definition.
(I've seen this before)
'pen used round old lines in' is the wordplay.
'pen' becomes 'cage' (I've seen this before).
'used round' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lines in' becomes 'll' (two abbreviations for line. I am not sure about the 'in' bit.).
'o'+'ll'='oll'
'cage' going around 'oll' is 'COLLAGE'.
(Other definitions for collage that I've seen before include "Stuck-on art form" , "Paste-up" , "Assembled-fragments art" , "Artwork formed from scraps of paper, photos etc" , "work being stuck" .)