Last target certain to be topped (6)
I believe the answer is:
endure
'last' is the definition.
(I know that last can be written as endure)
'target certain to be topped' is the wordplay.
'target' becomes 'end' (I've seen this before).
'certain' becomes 'sure' (synonyms).
'to be topped' means to remove the first letter (to top something is to remove its top).
'sure' with its first letter removed is 'ure'.
'end'+'ure'='ENDURE'
(Other definitions for endure that I've seen before include "Continue to exist" , "Go through" , "Put up with, say hardship" , "Bear, stick it out" , "Suffer or undergo" .)