Minister on valium initially (3)
I believe the answer is:
rev
'minister' is the definition.
(short for Reverend, a church minister)
'on valium initially' is the wordplay.
'on' becomes 're' (regarding).
'initially' suggests taking the first letters.
The initial letter of 'valium' is 'v'.
're'+'v'='REV'
(Other definitions for rev that I've seen before include "One turn of an engine, in short" , "Clergyman, or woman (abbr)" , "Motor rotation, in short" , "serviceman?" , "Reverend (abbr.)" .)