Possible reason for extra head on everyone (2,4)
I believe the answer is:
no ball
'possible reason for extra' is the definition.
(cricket)
'head on everyone' is the wordplay.
'head' becomes 'nob' (I've seen this in another clue).
'on' means one lot of letters go next to another.
'everyone' becomes 'all' (all people).
'nob'+'all'='NO-BALL'
(Other definitions for no ball that I've seen before include "Extra in cricket" , "Illegal delivery" , "Botched delivery" , "delivery's not good" , "Disallowed delivery" .)