Demolish last of carrots and potatoes (5)
I believe the answer is:
smash
'demolish' is the definition.
(thesaurus)
'last of carrots and potatoes' is the wordplay.
'last of' suggests the final letters.
'and' says to put letters next to each other.
'potatoes' becomes 'mash' (I've seen this before).
The final letter of 'carrots' is 's'.
's'+'mash'='SMASH'
(Other definitions for smash that I've seen before include "Shatter, break up" , "Hard tennis shot" , "Overarm volley" , "tremendous success!" , "Overhead tennis shot" .)