Too large to implement (4)
I believe the answer is:
tool
'to implement' is the definition.
(tool is a kind of implement)
'too large' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'too'+'l'='TOOL'
(Other definitions for tool that I've seen before include "Pawn" , "Device used for work" , "Instrument, implement" , "Hand implement" , "organ" .)