Venomous snake count discovered in steam engine (4,5)
I believe the answer is:
puff adder
'venomous snake' is the definition.
'count discovered in steam engine' is the wordplay.
'count' becomes 'add' (adding is a kind of counting).
'discovered in' is an insertion indicator.
'steam engine' becomes 'puffer'.
'add' put into 'puffer' is 'PUFF-ADDER'.
(Other definitions for puff adder that I've seen before include "Venomous viper" , "Large viper" , "poisonous creeper" , "Poisonous type" , "Snake" .)