Spider's back in cross-section (4)
I believe the answer is:
trap
'spider's back' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'cross-section' is the wordplay.
'cross' becomes 'T' (shape).
'section' becomes 'rap' (I can't justify this - if you can you should believe this answer much more).
't'+'rap'='TRAP'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for trap that I've seen before include "Ambush" , "mug" , "Kisser" , "Driver's obstacle" , "that may be sprung?" .)