About to kiss a boy (3)
I believe the answer is:
rex
'a boy' is the definition.
'about to kiss' is the wordplay.
'about' becomes 're' (regarding).
'to kiss' becomes 'x'.
're'+'x'='REX'
(Other definitions for rex that I've seen before include "King's title" , "One in the palace" , "King (Latin); cat breed" , "King (L.)" , "See 6" .)