Excessively large instrument (4)
I believe the answer is:
tool
'instrument' is the definition.
(I've seen this before)
'excessively large' is the wordplay.
'excessively' becomes 'too' ('too' can be a synonym of 'excessively').
'large' becomes 'l'.
'too'+'l'='TOOL'
(Other definitions for tool that I've seen before include "Device, implement" , "Hand implement" , "Working instrument" , "Maybe saw" , "Weapon" .)