Vicar turning irrational, probing apparition in great excitement (5,5)
I believe the answer is:
Here is my best explanation:
'great excitement' is the definition.
'vicar turning irrational probing apparition' is the subsidiary indication.
'vicar' becomes 'rev'.
'turning' says the letters should be written in reverse.
'irrational' becomes 'pi' (an irrational number).
'probing' is an insertion indicator.
'apparition' becomes 'fetch' (type of ghost or apparition).
'rev' reversed gives 'ver'.
'verpi' placed within 'fetch' is 'feverpitch'.
'in' is the link.
(Other definitions for fever pitch that I've seen before include "State of extreme excitement", "State of intense excitement - fetch viper (anag)", "Height of excitement" and "State of high excitement".)