Book by scientists about oxygen on island (6)
I believe the answer is:
romans
'book' is the definition.
(I know that Romans is a Epistle)
'scientists about oxygen on island' is the wordplay.
'scientists' becomes 'rs' (Royal Society).
'about' indicates putting letters inside.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'on' is a charade indicator (letters next to each other).
'island' becomes 'man' (the Isle of Man is an example).
'o'+'man'='oman'
'rs' going around 'oman' is 'ROMANS'.
'by' acts as a link.
(Other definitions for romans that I've seen before include "'Friends, . . . . . ., countrymen, lend me your ears' (Julius Caesar)" , "Ancient Italians" , "New Testament book" , "Mark Antony's countrymen?" , "Classical Italians" .)