Walls of extension will have this appearance, if you can picture it (8)
I believe the answer is:
envisage
'you can picture it' is the definition.
I can't judge whether this definition defines the answer.
'walls of extension will have this appearance' is the wordplay.
'walls of' says to hollow out the word (remove centre letters) (only the edge letters remain).
'will have' says to put letters next to each other.
'this appearance' becomes 'visage' (visage is a kind of appearance).
'extension' with its middle removed is 'en'.
'en'+'visage'='ENVISAGE'
'if' is the link.
Can you help me to learn more?
(Other definitions for envisage that I've seen before include "Picture in one's mind" , "Mentally picture" , "Consider, contemplate" , "Visualise, imagine" , "Conceive of, imagine" .)