Half of them run when hearts beat (6)
I believe the answer is:
thrash
'beat' is the definition.
(thrashing is a kind of beating)
'half of them run when hearts' is the wordplay.
'half of' suggests halving the letters.
'run' becomes 'r' (cricket abbreviation).
'when' becomes 'as'.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'them' halved is 'th'.
'th'+'r'+'as'+'h'='THRASH'
(Other definitions for thrash that I've seen before include "Beat; party (colloq.)" , "Whip repeatedly" , "wild party" , "Wallop" , "worst > party" .)