Bad clue about heartless dog in club (6)
I believe the answer is:
cudgel
'club' is the definition.
(cudgel is a kind of club)
'bad clue about heartless dog' is the wordplay.
'bad' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'heartless' means to remove the middle letters.
'dog' with its middle taken out is 'dg'.
'clue' anagrammed gives 'cuel'.
'cuel' going around 'dg' is 'CUDGEL'.
'in' acts as a link.
(Other definitions for cudgel that I've seen before include "Heavy staff used as a weapon" , "Heavy staff, club" , "Bludgeon" , "A ruffian's club" , "Blunt instrument" .)