Reptile took first steps (5)
I believe the answer is:
adder
'reptile' is the definition.
(adder is a kind of reptile)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for adder that I've seen before include "venomous creature" , "1 of 21 [SNAKES]" , "British snake; summer" , "Venomous sort" , "danger in the countryside" .)