Sponge in complimentary pack (8)
I believe the answer is:
freeload
'sponge' is the definition.
(freeloading is a kind of sponging)
'complimentary pack' is the wordplay.
'complimentary' becomes 'free' ('free' can be a synonym of 'complimentary').
'pack' becomes 'load' (loading is a kind of packing).
'free'+'load'='FREELOAD'
'in' is the link.
(Other definitions for freeload that I've seen before include "Bum" , "Without giving anything in return, take" , "Sponge" , "Scrounge" , "Enjoy oneself at another's expense" .)