Retired businessman penning tense book (5)
I believe the answer is:
titus
'book' is the definition.
(the Epistle of Paul to Titus, a book of the Bible)
'retired businessman penning tense' is the wordplay.
'retired' shows that the letters should be reversed in order.
'businessman' becomes 'suit' (I've seen this in another clue).
'penning' means one lot of letters goes inside another.
'tense' becomes 't' (abbreviation used in many dictionaries).
'suit' placed around 't' is 'sutit'.
'sutit' reversed gives 'TITUS'.
(Other definitions for titus that I've seen before include ". . . . . Andronicus, Shakespearean tragedy" , "-- Groan, novel by Mervyn Peake" , "A Roman emperor; a Pauline addressee" , "- Andronicus (Shak.)" , "Groan, say" .)