Spike's grown up with eye-opening speed (10)
I believe the answer is:
adulterate
'spike's' is the definition.
(thesaurus)
'grown up with eye-opening speed' is the wordplay.
'grown up' becomes 'adult' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'opening' says to take the initial letters.
'speed' becomes 'rate' (rate is a kind of speed**).
The initial letter of 'eye' is 'e'.
'adult'+'e'+'rate'='ADULTERATE'
(Other definitions for adulterate that I've seen before include "Reduce purity" , "Make impure by addition" , "Spoil" , "Doctor" , "Debase by adding inferior material" .)