It doesn't weaken the tree (3)
I believe the answer is:
sap
'weaken the tree' is the definition.
'sap' can be an answer for 'weaken' (I've seen this before). I'm unsure of the 'the tree' bit.
'it doesn't' is the wordplay.
I cannot quite see how this works, but
'it' could be 'sa' (SA can mean sex appeal, as in 'it') and 'sa' is found within the answer.
A single letter 'p' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sap that I've seen before include "Gradually weaken (a person's strength)" , "Plant juice for silly ass" , "Tree's blood" , "Vigour, energy" , "One is easily fooled" .)