Make an illustration using old pen (7)
I believe the answer is:
expound
'make an illustration using old pen' is the definition.
I can't judge whether this definition defines the answer.
'old pen' is the wordplay.
'old' becomes 'ex'.
'pen' becomes 'pound' (I've seen this before).
'ex'+'pound'='EXPOUND'
Can you help me to learn more?
(Other definitions for expound that I've seen before include "Set forth, explain in detail" , "Elucidate" , "Put forward detailed argument" , "Spell out" , "Elaborate" .)