Fine weapon -- Old MacDonald had one (4)
I believe the answer is:
farm
'old macdonald had one' is the definition.
(I've seen this in another clue)
'fine weapon' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'weapon' becomes 'arm' (synonyms).
'f'+'arm'='FARM'
(Other definitions for farm that I've seen before include "Use the land" , "Country workplace" , "Rural establishment" , "See 5" , "Homestead" .)