Garden pest's not left mysterious tropical grass (5,4)
I believe the answer is:
sugar cane
'grass' is the definition.
(I've seen this before)
'garden pest's not left mysterious' is the wordplay.
'garden pest' becomes 'slug' (I've seen this before).
'not' suggests deleting specific letters.
'left' becomes 'l' (common abbreviation).
'mysterious' becomes 'arcane' (I've seen this in another clue).
'slug' with 'l' taken out is 'sug'.
'sug'+'arcane'='SUGAR-CANE'
'tropical' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sugar cane that I've seen before include "supergrass?" , "Source of sweet stuff" , "Source of sweetener" , "What makes a spirit" , "Grass" .)