Old letters turn up in the morning (4)
I believe the answer is:
ogam
'old letters' is the definition.
(I've seen this before)
'turn up in the morning' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in the morning' becomes 'am' (short for ante meridiem - before noon).
'go' in reverse letter order is 'og'.
'og'+'am'='OGAM'
(Other definitions for ogam that I've seen before include "old letters" , "inscription" , "Old character" , "Ancient Celtic alphabet" .)