Implement excessively large (4)
I believe the answer is:
tool
'implement' is the definition.
(I know that implement can be written as 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 "Instrument, implement" , "Implement - agency" , "Maybe saw" , "Person exploited" , "organ" .)