Happens to make harvests better (5,2)

I believe the answer is:

crops up

I'm a little stuck... Click here to teach me more about this clue!

'happens to' is the definition.
Although both the answer and definition are verbs in their -s form, I can't understand how they can define each other.

'harvests better' is the wordplay.
'harvests' becomes 'crops' ('crop' can be a synonym of 'harvest').
'better' becomes 'up' (I can't justify this - if you can you should believe this answer much more).
'crops'+'up'='CROPS UP'

'make' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app