Defect from regiment at last, post-conflict (4)
I believe the answer is:
wart
'defect' is the definition.
(wart is a kind of defect)
'regiment at last post-conflict' is the wordplay.
'at last' says to take the final letters.
'post' means one lot of letters go next to another (post- can mean after).
'conflict' becomes 'war' (war is a kind of conflict).
The last letter of 'regiment' is 't'.
't' after 'war' is 'WART'.
'from' acts as a link.
(Other definitions for wart that I've seen before include "Small rounded protuberance on the skin" , "Charmable excrescence" , "Small hard excrescence on the skin" , "Abnormal blemish on the skin, caused by a virus" , "Unsightly skin growth" .)