Small insect, a visitor in December (5)
I believe the answer is:
santa
'visitor in december' is the definition.
'santa' can be an answer for 'visitor' (I have seen 'generous visitor' mean 'santa' so perhaps 'visitor' could also mean 'santa'). I am not sure about the 'in december' bit.
'small insect a' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'insect' becomes 'ant' (ant is a kind of insect).
's'+'ant'+'a'='SANTA'
Can you help me to learn more?
(Other definitions for santa that I've seen before include "Dad in a red and white outfit?" , "Midwinter gift bringer" , "He comes down chimney" , "The present chap" , "A children's favourite may" .)