Ball in the box pushed back Hearts regularly - they put a cross in the box (6)
I believe the answer is:
voters
'they put a cross in the box' is the definition.
The definition suggests an adverb but the answer is not.
'ball in the box pushed back hearts regularly' is the wordplay.
'ball' becomes 'o' (O looks like a ball).
'in' is an insertion indicator.
'the box' becomes 'tv' ().
'pushed back' says the letters should be written in reverse.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'hearts' are 'ers'.
'o' put inside 'tv' is 'tov'.
'tov' in reverse letter order is 'vot'.
'vot'+'ers'='VOTERS'
Can you help me to learn more?
(Other definitions for voters that I've seen before include "They cast ballots" , "Selectors" , "Electors" .)