Vicar turning irrational, probing apparition in great excitement (5,5)
I believe the answer is:
fever pitch
'great excitement' is the definition.
'vicar turning irrational probing apparition' is the wordplay.
'vicar' becomes 'rev'.
'turning' says the letters should be written backwards.
'irrational' becomes 'pi' (an irrational number).
'probing' is an insertion indicator (probe can mean to make a hole in).
'apparition' becomes 'fetch' (type of ghost or apparition).
'rev' back-to-front is 'ver'.
'ver'+'pi'='verpi'
'verpi' placed inside 'fetch' is 'FEVER PITCH'.
'in' is the link.
(Other definitions for fever pitch that I've seen before include "state of great excitement" , "State of extreme excitement" , "in state of great agitation" , "Tremendous excitement" , "State of intense excitement - fetch viper (anag)" .)