Vicar up against ecclesiastical head’s pizzazz (5)
I believe the answer is:
verve
'pizzazz' is the definition.
(I've seen this in another clue)
'vicar up against ecclesiastical head's' is the wordplay.
'vicar' becomes 'rev' (reverend).
'up' is a reversal indicator (in down clue: letters go upwards).
'against' becomes 'v' (abbreviation for 'versus').
'head's' suggests taking the first letters (I've seen 'head' mean this).
The first letter of 'ecclesiastical' is 'e'.
'rev' written backwards gives 'ver'.
'ver'+'v'+'e'='VERVE'
(Other definitions for verve that I've seen before include "Energy and zest" , "Sparkling enthusiasm" , "zip" , "Animation" , "Gusto; energy" .)