Finch, small one next to hide (6)
I believe the answer is:
siskin
'finch' is the definition.
(siskin is a kind of finch)
'small one next to hide' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'one' becomes 'i' (Roman numeral).
'next to' means one lot of letters go next to another.
'hide' becomes 'skin' ('skin' can be a synonym of 'hide'**).
's'+'i'+'skin'='SISKIN'
(Other definitions for siskin that I've seen before include "Yellow-green bird" , "Greenish songbird of finch family" , "Finch akin to canary" , "Singer" , "Yellow-green finch" .)