Bung in small hat (7)
I believe the answer is:
stopper
'bung' is the definition.
(thesaurus)
'small hat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'hat' becomes 'topper' (topper is a kind of hat).
's'+'topper'='STOPPER'
'in' is the link.
(Other definitions for stopper that I've seen before include "Bung" , "One who checks" , "One blocks" , "Perhaps cork" , "Cork - goalkeeper?" .)