Slight expert on theological matters (6)
I believe the answer is:
rebuff
'slight' is the definition.
(I know that slight can be written as rebuff)
'expert on theological matters' is the wordplay.
I cannot quite understand how this works, but
'expert' could be 'buff' (buff can mean an expert or aficionado) and 'buff' is present in the answer.
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rebuff that I've seen before include "Ungracious rejection of a friendly approach" , "Drive back" , "Snub, blunt refusal" , "Check" , "Snub or contemptuous refusal" .)