Clue: Challenge one in bed – early riser? (4,5)
Answer: TEST PILOT
Explanation: Definition: early riser
 TEST(to challenge/to check the characteristics of something) + [ I(Roman numeral for "one") **contained in**(in) PLOT(a bed/an area of ground) ].
Defn: The first, or one of the first, to fly/rise above in a new aircraft, cryptically described as an …