Tropical plant in sludge oddly hard to grasp (5,4)
I believe the answer is:
sugar cane
'tropical plant' is the definition.
'sugar cane' can be an answer for 'plant' (sugar cane is a kind of plant). I'm not sure about the 'tropical' bit.
'sludge oddly hard to grasp' is the wordplay.
'oddly' means one should take alternating letters (letters in odd-numbered positions).
'hard to grasp' becomes 'arcane' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The alternate letters of 'sludge' are 'sug'.
'sug'+'arcane'='SUGAR-CANE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for sugar cane that I've seen before include "Grass" , "supergrass?" , "What makes a spirit" , "Source of sweetener" , "Source of sweet stuff" .)