In board game, a possible way down when way up’s a non-starter (5)
I believe the answer is:
adder
'in board game a possible way down' is the definition.
(a snake in the game Snakes and Ladders)
'way up's a non-starter' is the wordplay.
'way up' becomes 'ladder' (eg the way up to an attic).
'a non-starter' means to remove the first letter (the letter which starts the word is ignored).
'ladder' with its initial letter taken off is 'ADDER'.
'when' acts as a link.
(Other definitions for adder that I've seen before include "1 of 21 [SNAKES]" , "a poisonous type" , "Poisonous Brit. snake" , "One contributing more" , "poisoner" .)