Idiots play sport on Sunday (5)
I believe the answer is:
dopes
'idiots' is the definition.
'play sport on sunday' is the wordplay.
'play' becomes 'do' (both can mean to perform something).
'sport' becomes 'pe' (physical education).
'on' means one lot of letters go next to another.
'sunday' becomes 's'.
'do'+'pe'+'s'='DOPES'
(Other definitions for dopes that I've seen before include "Nobbles" , "Fools" .)