Those last seen in militia will attack Area 51 base (6)
I believe the answer is:
alkali
'base' is the definition.
(synonyms)
'those last seen in militia will attack area 51' is the wordplay.
'those last seen in' suggests the final letters (the last letters seen in the words).
'51' becomes 'LI' (roman numeral).
The final letters of 'militia will attack area' are 'alka'.
'alka'+'li'='ALKALI'
(Other definitions for alkali that I've seen before include "Compound like lime" , "Chemical base" , "In pH terms, not acid" , "bases" , "Substance that neutralises acid" .)