Old chap enthralled by cover of religious book (6)
I believe the answer is:
romans
'book' is the definition.
(I've seen this before)
'old chap enthralled by cover of religious' is the wordplay.
'old' becomes 'o' (abbreviation).
'chap' becomes 'man' (chap can mean a man).
'enthralled by' indicates putting letters inside ('enthral' can mean 'capture').
'cover of' means to remove the middle letters (only the letters which cover the word).
'religious' with its centre removed is 'rs'.
'o'+'man'='oman'
'oman' going inside 'rs' is 'ROMANS'.
(Other definitions for romans that I've seen before include "Citizens of Italian capital" , "Classical Italians" , "Mark Antony's countrymen?" , "New Testament book" , "'Friends, . . . . . ., countrymen, lend me your ears' (Julius Caesar)" .)