Charge added to complimentary sponge (8)
I believe the answer is:
freeload
'sponge' is the definition.
(freeloading is a kind of sponging)
'charge added to complimentary' is the wordplay.
'charge' becomes 'load' ('load' can be a synonym of 'charge'**).
'added to' is a charade indicator (letters next to each other).
'complimentary' becomes 'free' ('free' can be a synonym of 'complimentary').
'load' put after 'free' is 'FREELOAD'.
(Other definitions for freeload that I've seen before include "Enjoy oneself at another's expense" , "Accept generosity while giving nothing in return" , "Bum" , "Live off somebody's generosity" , "Scrounge" .)