Fit tree into small container (8)
I believe the answer is:
matchbox
'small container' is the definition.
'fit tree' is the wordplay.
'fit' becomes 'match' ('match' can be a synonym of 'fit').
'tree' becomes 'box' (box tree is a type of tree).
'match'+'box'='MATCHBOX'
'into' is the link.
(Other definitions for matchbox that I've seen before include "This will hold your vestas" , "that has inflammable contents?" , "Container for lighters" , "Small" , "in which strikers are contained" .)