For starters, vicar improvising sermon has never understood God (6)
I believe the answer is:
vishnu
'god' is the definition.
(Vishnu is an example)
'for starters vicar improvising sermon has never understood' is the wordplay.
'for starters' says to take the initial letters.
The first letters of 'vicar improvising sermon has never understood' is 'VISHNU'.
(Other definitions for vishnu that I've seen before include "Saviour god of Hinduism" , "Hindu 'Preserver' god" , "Hindu divinity" , "Hindu deity, the Preserver" , "Important Hindu god" .)