He is back on Monday (5)
I believe the answer is:
simon
'he' is the definition.
'is back on monday' is the wordplay.
'back' says the letters should be written backwards.
'on' says to put letters next to each other.
'monday' becomes 'Mon' (short for Monday).
'is' in reverse letter order is 'si'.
'si'+'mon'='SIMON'
(Other definitions for simon that I've seen before include "Simple pieman" , "Man" , "- and Garfunkel" , "He's simple in rhyme" , "Apostle Peter's other name" .)