Hearts directly affected with core of flower power (13)
I believe the answer is:
hydroelectric
'flower power' is the definition.
(Power from something that flows: a flow-er)
'hearts directly affected with core' is the wordplay.
'hearts' becomes 'h' (abbreviation used in card games e.g. bridge).
'affected with' indicates anagramming the letters.
'directly' with its letters anagrammed with 'core' makes 'ydroelectric'.
'h'+'ydroelectric'='HYDROELECTRIC'
'of' acts as a link.
(Other definitions for hydroelectric that I've seen before include "Power generated by water" , "Like water-driven power stations" , "Sort of power" , "Of the generation of power using flowing water" .)