One against endless search for period pieces (8)
I believe the answer is:
antiques
'period pieces' is the definition.
(I've seen this before)
'one against endless search' is the wordplay.
'one against' becomes 'anti' (prefix meaning against).
'endless' means to remove the last letter.
'search' becomes 'quest' (quest is a kind of search).
'quest' with its final letter taken off is 'ques'.
'anti'+'ques'='ANTIQUES'
'for' acts as a link.
(Other definitions for antiques that I've seen before include "Old collectables" , "Collectable old objects" , "period pieces" , "valuable stuff" , "Objects grown valuable with age" .)