Hammer perhaps excessively large (4)
I believe the answer is:
tool
'hammer perhaps' is the definition.
(I know that hammer is a type of impact tool)
'excessively large' is the wordplay.
'excessively' becomes 'too' (to too great a degree).
'large' becomes 'L' (eg in clothes sizes).
'too'+'l'='TOOL'
(Other definitions for tool that I've seen before include "Appliance" , "Practical implement" , "Person exploited" , "Device, implement" , "Hand implement" .)