Pet about to sit on head (8)
I believe the answer is:
canoodle
'pet' is the definition.
(both can mean to touch lovingly)
'about to sit on head' is the wordplay.
'about' becomes 'ca' (circa).
'to sit on' says to put letters next to each other (in a down clue, some letters go on top of others).
'head' becomes 'noodle' (noodle is a kind of head).
'ca'+'noodle'='CANOODLE'
(Other definitions for canoodle that I've seen before include "Hug and kiss" , "Behave amorously" , "Fondle" , "Kiss and cuddle (informal)" , "Pet" .)