Within litter, hearts beat (6)
I believe the answer is:
thrash
'beat' is the definition.
(I know that thrash is a more specific form of the action beat)
'within litter hearts' is the wordplay.
'within' means one lot of letters goes inside another.
'litter' becomes 'trash' (I've seen this in another clue).
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'trash' enclosing 'h' is 'THRASH'.
(Other definitions for thrash that I've seen before include "Beat violently" , "worst > party" , "wild party" , "Drub" , "Clobber" .)