Rate terribly evil toy entertaining child at first (8)
I believe the answer is:
velocity
'rate' is the definition.
(velocity is a kind of rate)
'terribly evil toy entertaining child at first' is the wordplay.
'terribly' indicates an anagram.
'entertaining' is an insertion indicator.
'at first' says to take the initial letters.
The initial letter of 'child' is 'c'.
'evil'+'toy'='eviltoy'
'eviltoy' anagrammed gives 'veloity'.
'veloity' placed around 'c' is 'VELOCITY'.
(Other definitions for velocity that I've seen before include "Agreeable -- Riviera resort" , "Swiftness" , "Speed of travel" , "Love city on speed" , "Rate of motion in a specified direction" .)