First guy had a bad back by morning (4)
I believe the answer is:
adam
'first guy' is the definition.
(I've seen this before)
'a bad back by morning' is the wordplay.
'back' says to take the final letters (the letter at the back).
'by' means one lot of letters go next to another.
'morning' becomes 'am' (as in 12:00 AM).
The final letter of 'bad' is 'd'.
'a'+'d'+'am'='ADAM'
'had' acts as a link.
(Other definitions for adam that I've seen before include "Father of Cain and Abel" , "expelled gardener" , "Eden inhabitant" , "God's first son" , "one might confuse with total stranger" .)