Head kissing has no boundaries (6)
I believe the answer is:
noggin
'head' is the definition.
(noggin is a kind of head)
'kissing has no boundaries' is the wordplay.
'kissing' becomes 'snogging' (snogging is a kind of kissing).
'has no boundaries' means to look at the middle letters (the letters at the boundaries are removed).
The centre of 'snogging' is 'NOGGIN'.
(Other definitions for noggin that I've seen before include "Small measure of liquid" , "Small mug - wooden cup" , "Gill usually" , "Small measure of spirits or slangy head" , "Small quantity of spirits" .)