Small kid in long blouse (5)
I believe the answer is:
smock
'long blouse' is the definition.
'small kid' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'kid' becomes 'mock'.
's'+'mock'='SMOCK'
'in' acts as a link.
(Other definitions for smock that I've seen before include "Loose dress or blouse" , "Loose outer garment" , "Artist's, peasant's, overall" , "Loose overall or dress" , "artist's attire" .)