Remains in middle of Area 51 close to disc (5)
I believe the answer is:
relic
'remains' is the definition.
(I've seen this before)
'middle of area 51 close to disc' is the wordplay.
'middle of' means to look at the middle letters.
'51' becomes 'LI' (roman numeral).
'close to' says to take the final letters (the 'close' to the word).
The central letters of 'area' are 're'.
The last letter of 'disc' is 'c'.
're'+'li'+'c'='RELIC'
'in' is the link.
(Other definitions for relic that I've seen before include "Ancient and venerated object" , "Holy remnant" , "Surviving ancient custom" , "(Holy) remains" , "Heirloom" .)