High jumper in fine meadow (4)
I believe the answer is:
flea
'high jumper' is the definition.
(I've seen this in another clue)
'fine meadow' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'meadow' becomes 'lea' (a lea is a grassy area).
'f'+'lea'='FLEA'
'in' is the link.
(Other definitions for flea that I've seen before include "Small jumper in small circus" , "beastie?" , "Small blood-sucking insect" , "Bloodsucking parasite" , "Jumping parasite" .)