Oppose HST after understanding land’s cut by line (9)
I believe the answer is:
withstand
'oppose' is the definition.
(withstanding is a kind of opposing)
'hst after understanding land's cut by line' is the wordplay.
'after' says to put letters next to each other.
'understanding' becomes 'wit'.
'cut by' suggests deleting specific letters.
'line' becomes 'l' (used when specifying particular lines from a poem).
'land' with 'l' removed is 'and'.
'hst' after 'wit' is 'withst'.
'withst'+'and'='WITHSTAND'
(Other definitions for withstand that I've seen before include "Show fight" , "Bear" , "Resist, hold out" , "Resist or brave something" , "Remain undamaged by" .)