Snake now turns up red (5)
I believe the answer is:
adder
'snake' is the definition.
(adder is a kind of snake)
'now turns up red' is the wordplay.
'now' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'turns up' is a reversal indicator.
'red' backwards is 'der'.
'ad'+'der'='ADDER'
(Other definitions for adder that I've seen before include "1 of 21 [SNAKES]" , "Mathematical reptile" , "exhibit in the reptile house" , "Viper common in northern Eurasia" , "He's making an extension" .)