Cosmo doctored front of Heidi's neck (6)
I believe the answer is:
smooch
'neck' is the definition.
(smooching is a kind of necking)
'cosmo doctored front of heidi's' is the wordplay.
'doctored' indicates anagramming the letters.
'front of' says to take the initial letters (the letter at the front of the word).
The initial letter of 'heidi' is 'h'.
'cosmo' with letters rearranged gives 'smooc'.
'smooc'+'h'='SMOOCH'
(Other definitions for smooch that I've seen before include "Snuggle" , "Kiss and caress slangily" , "Canoodle" , "Dance slowly while in an embrace - kiss - pet" , "neck" .)