Botanist leaves, having gathered agave fibre (5)
I believe the answer is:
istle
'agave fibre' is the definition.
(fibre made from agave plants)
'botanist leaves having gathered' is the wordplay.
'having gathered' says the answer is hidden in the clue.
'ISTLE' is hidden within 'botanist leaves'.
(Other definitions for istle that I've seen before include "Carpet material" , "for making nets?" , "Fibre from yucca or agave trees" .)