Happen to harvest more (4,2)
I believe the answer is:
crop up
'happen' is the definition.
(I've seen this before)
'harvest more' is the wordplay.
'harvest' becomes 'crop' (synonyms).
'more' becomes 'up' (I can't justify this - if you can you should give a lot more credence to this answer).
'crop'+'up'='CROP UP'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for crop up that I've seen before include "Appear unexpectedly" , "Occur by chance - like harvest?" , "Happen" , "Occur unexpectedly - like harvest?" , "Occur, arise" .)