Small shelf for hammer (6)
I believe the answer is:
sledge
'hammer' is the definition.
(sledge is a kind of hammer)
'small shelf' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'shelf' becomes 'ledge' ('ledge' can be a synonym of 'shelf').
's'+'ledge'='SLEDGE'
'for' is the link.
(Other definitions for sledge that I've seen before include "Snow conveyance" , "Vehicle for use in snow" , "Vehicle for transportation over snow" , "Vehicle on runners, for snow and ice" , "Vehicle mounted on runners" .)