One’s left with three kisses on his birthday? (12)
I believe the answer is:
octogenarian
'his birthday?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one's left with three kisses on' is the wordplay.
I cannot really understand how this works, but
'on' could be 'to' and 'to' is found within the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
This explanation may well be incorrect...
(Other definitions for octogenarian that I've seen before include "One aged 80 to 89" , "Augustus John in last years" , "Person over 80" , "One in their eighties" , "Someone aged between 80 and 89" .)