With three copies, Penny left one out, which is complex (9)
I believe the answer is:
intricate
'is complex' is the definition.
(similar in meaning)
'with three copies penny left one out' is the wordplay.
I cannot really understand how this works, but
'three' could be 'tri' (prefix meaning 'three') and 'tri' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
This explanation may well be incorrect...
'which' acts as a link.
Can you help me to learn more?
(Other definitions for intricate that I've seen before include "Very complicated and involved" , "ornate" , "puzzlingly obscure" , "Complex, labyrinthine" , "Complicated, detailed" .)