Book by Royal Society featuring old fellow (6)
I believe the answer is:
romans
'book' is the definition.
(I've seen this before)
'royal society featuring old fellow' is the wordplay.
'royal society' becomes 'rs'.
'featuring' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fellow' becomes 'man' (I've seen this before).
'o'+'man'='oman'
'rs' placed 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)" , "Mark Antony's countrymen?" , "Empire builders" , "Ancient Italians" , "New Testament book" .)