Mark part of bible to introduce round church (5)
I believe the answer is:
notch
'mark' is the definition.
(I've seen this before)
'part of bible to introduce round church' is the wordplay.
'part of bible' becomes 'nt' (New Testament abbreviation).
'to introduce' means one lot of letters goes inside another (to introduce to can mean to insert or include).
'round' becomes 'o' (round can mean a round object).
'church' becomes 'ch' (common abbreviation).
'nt' placed around 'o' is 'not'.
'not'+'ch'='NOTCH'
(Other definitions for notch that I've seen before include "Secure" , "Nick - incision" , "Snick" , "Point or degree in a scale" , "Make the cut" .)