Small insect on a Christmas visitor (5)
I believe the answer is:
santa
'christmas visitor' is the definition.
(I've seen this before)
'small insect on a' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'insect' becomes 'ant' (ant is a kind of insect).
'on' means one lot of letters go next to another.
's'+'ant'+'a'='SANTA'
(Other definitions for santa that I've seen before include "Dad in a red and white outfit?" , "He comes down chimney" , "-- Barbara" , "& 22 Father Christmas" , "Present bringer" .)