Largest threat to fruit crop? (8)
I believe the answer is:
plumpest
'largest' is the definition.
(plump can mean large)
'fruit crop?' is the wordplay.
'fruit' becomes 'plum' (plum is a kind of fruit).
'crop?' becomes 'pest' (I can't explain this - if you can you should believe this answer much more).
'plum'+'pest'='PLUMPEST'
'threat to' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Another definition for plumpest that I've seen is " better fed than the rest?".)