Surprise a holy man on his involvement (8)
I believe the answer is:
astonish
'surprise' is the definition.
(astonishing is a kind of surprising)
'a holy man on his involvement' is the wordplay.
'holy man' becomes 'st' (abbreviation for saint).
'involvement' indicates anagramming the letters.
'his' is an anagram of 'ish'.
'a'+'st'+'on'+'ish'='ASTONISH'
(Other definitions for astonish that I've seen before include "are staggering" , "Provoke wonder" , "Gobsmack" , "Greatly impress" , "Stun" .)