Young bee piercing brown horse (4)
I believe the answer is:
baby
'young' is the definition.
(baby is a kind of young)
'bee piercing brown horse' is the wordplay.
'bee' becomes 'b'.
'piercing' means one lot of letters goes inside another (some letters pierce a hole in a word).
'brown horse' becomes 'bay' (reddish-brown horse coat colour).
'b' going within 'bay' is 'BABY'.
(Other definitions for baby that I've seen before include "Very young child" , "Timid person; very small" , "Love" , "New-born child" , "Infant" .)