Cleric, very holy within? The reverse — a venomous type! (5)
I believe the answer is:
viper
'a venomous type' is the definition.
'cleric very holy within? the reverse' is the wordplay.
'cleric' becomes 'rev' (abbreviation for Reverend).
'very holy' becomes 'pi' (old-fashioned slang for 'pious').
'within?' is an insertion indicator.
'the reverse' shows that the letters should be reversed in order.
'rev' placed around 'pi' is 'repiv'.
'repiv' in reverse letter order is 'VIPER'.
(Other definitions for viper that I've seen before include "A small poisonous snake" , "Venomous snake, person" , "Adder" , "crawler" , "spiteful person" .)