Kiss that's slightly under par (6)
I believe the answer is:
birdie
'slightly under par' is the definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe there's a link between them I don't understand?
'kiss that's' is the wordplay.
'kiss' becomes 'bird' (I can't explain this - if you can you should give a lot more credence to this answer).
'that's' becomes 'ie' (abbreviation for 'id est', 'that is' in Latin).
'bird'+'ie'='BIRDIE'
Can you help me to learn more?
(Other definitions for birdie that I've seen before include "One watched" , "Golf score of one stroke under par" , "Good hole" , "low score" , "In golf, a score of one under par for the hole" .)