Middle of plant yielded sisal, say (5)
I believe the answer is:
agave
'sisal say' is the definition.
(sisal is a kind of agave)
'middle of plant yielded' is the wordplay.
'middle of' means to look at the middle letters.
'yielded' becomes 'gave' ('give' can be a synonym of 'yield').
The middle letter of 'plant' is 'a'.
'a'+'gave'='AGAVE'
(Other definitions for agave that I've seen before include "native growing up in tropical America" , "Spiky succulent" , "A cactus" , "Tropical plant" , "plant in Mexico" .)