Secrecy in the last assembly (7)
I believe the answer is:
stealth
'secrecy' is the definition.
(I've seen this before)
'the last assembly' is the wordplay.
'assembly' is an anagram indicator.
'the'+'last'='thelast'
'thelast' with letters rearranged gives 'STEALTH'.
'in' is the link.
(Other definitions for stealth that I've seen before include "word to define secrecy" , "Secretiveness, furtiveness" , "By which taxes go up" , "Cats display this" , "underhand dealing" .)