Cadet's last post idea works (5)
I believe the answer is:
plant
'works' is the definition.
(synonyms)
'cadet's last post idea' is the wordplay.
'last' suggests the final letters.
'post' says to put letters next to each other (post- can mean after).
'idea' becomes 'plan' (plan is a kind of idea).
The last letter of 'cadet' is 't'.
't' put after 'plan' is 'PLANT'.
(Other definitions for plant that I've seen before include "Factory; organism" , "One placed in a group as an informer" , "Member of vegetable kingdom" , "sold by garden centre" , "Spy; manufacturer's site" .)