Periodical served up piece as a good opener? (6)
I believe the answer is:
gambit
'a good opener?' is the definition.
(as in an opening gambit)
'periodical served up piece' is the wordplay.
'periodical' becomes 'mag' (abbreviation for magazine).
'served up' shows that the letters should be reversed in order (in a down clue, going upwards).
'piece' becomes 'bit' (bit is a kind of piece).
'mag' in reverse letter order is 'gam'.
'gam'+'bit'='GAMBIT'
'as' acts as a link.
(Other definitions for gambit that I've seen before include "tricky move" , "Opening chess move" , "Opening comment" , "Sacrificial strategy in chess" , "first move" .)