No joke, small island's flipping cactus (7)
I believe the answer is:
opuntia
'cactus' is the definition.
(I know that Opuntia is a type of cactus)
'no joke small island's flipping' is the wordplay.
'no' becomes 'O' (O resembles 0).
'joke' becomes 'pun' (pun is a type of joke).
'small island' becomes 'ait' (an ait is a small island in a river).
'flipping' says the letters should be written backwards.
'ait' backwards is 'tia'.
'o'+'pun'+'tia'='OPUNTIA'
(Other definitions for opuntia that I've seen before include "Eg, prickly pear" , "spiny plant" , "prickly plant" , "Genus of cacti" , "Prickly sort" .)