There's not even a chance this could pass for prickly pears (5)
I believe the answer is:
cacti
'prickly pears' is the definition.
(I've seen this in another clue)
'there's not even a chance this could pass' is the wordplay.
'there's' means one lot of letters go next to another.
'not even' means one should take alternating letters (remove even-numbered letters).
'this could pass' becomes 'ti' (I can't explain this - if you can you should give a lot more credence to this answer).
The alternate letters of 'chance' are 'cac'.
'cac' put next to 'ti' is 'CACTI'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for cacti that I've seen before include "Leafless succulents" , "Features of westerns" , "Prickly types" , "Tough plants" , "Prickly plants" .)