Measure the rain one has trapped (4)
I believe the answer is:
acre
'measure the rain' is the definition.
'acre' can be an answer for 'measure' (I've seen this before). I am unsure of the 'the rain' bit.
'one has trapped' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'has' means one lot of letters goes inside another.
'trapped' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'ace' going around 'r' is 'ACRE'.
Can you help me to learn more?
(Other definitions for acre that I've seen before include "Old unit of area - old port" , "Care about this bit of land" , "Ancient city in Israel" , "in Western Brazil" , "Race (anag.)" .)