Vespasian's son ____ was the first Roman emperor to succeed his biological father (5)
I believe the answer is:
titus
(Other definitions for titus that I've seen before include "good book" , "saint" , "Rom. emperor 79 to 81AD" , ". . . . . Andronicus, Shakespearean tragedy" , "-- Groan, novel by Mervyn Peake" .)