Rubber tail on baby's toy snake! (7)
I believe the answer is:
rattler
'snake' is the definition.
(rattler is a kind of snake)
'rubber tail on baby's toy' is the wordplay.
'tail' suggests the final letters.
'on' means one lot of letters go next to another.
'baby's toy' becomes 'rattle' (I've seen this before).
The final letter of 'rubber' is 'r'.
'r' after 'rattle' is 'RATTLER'.
(Other definitions for rattler that I've seen before include "A noisy American snake" , "North American pit viper" , "remarkably good specimen" , "US snake (colloq.)" , "I'm shaken!" .)