Priest is making initial crystal (5)
I believe the answer is:
prism
'crystal' is the definition.
(I've seen this before)
'priest is making initial' is the wordplay.
'priest' becomes 'pr'.
'initial' suggests taking the first letters.
The initial letter of 'making' is 'm'.
'pr'+'is'+'m'='PRISM'
(Other definitions for prism that I've seen before include "Female character in The Importance of Being Ernest -- optical device" , "Prims (anag)" , "It separates a beam of white light into colours" , "Transparent 3 sided object that seperates light into colours" , "Triangular glass shape, breaks up light" .)