A wizard after Potter's head, fast (5)
I believe the answer is:
apace
'fast' is the definition.
(apace can mean quickly)
'a wizard after potter's head' is the wordplay.
'wizard' becomes 'ace' (synonyms).
'after' says to put letters next to each other.
'head' suggests taking the first letters.
The initial letter of 'potters' is 'p'.
'ace' put after 'p' is 'pace'.
'a'+'pace'='APACE'
(Other definitions for apace that I've seen before include "Swiftly, quickly" , "(Move on) swiftly" , "Quick" , "fast" , "flat out" .)