Beats up less, possibly (6)
I believe the answer is:
pulses
'beats' is the definition.
('pulse' can be a synonym of 'beat')
'up less possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'up'+'less'='upless'
'upless' with letters rearranged gives 'PULSES'.
(Other definitions for pulses that I've seen before include "Expands and contracts rhythmically" , "Throbs" .)