He is back on Monday (5)
I believe the answer is:
simon
'he' is the definition.
'is back on monday' is the wordplay.
'back' shows that the letters should be reversed in order.
'on' is a charade indicator (letters next to each other).
'monday' becomes 'Mon' (short for Monday).
'is' written backwards gives '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" .)