Managed to get one to assess pluviometer (4,5)
I believe the answer is:
rain gauge
'pluviometer' is the definition.
(I've seen this in another clue)
'managed to get one to assess' is the wordplay.
'managed' becomes 'ran' (both can mean organised or was in charge of).
'to get' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'to assess' becomes 'gauge' (I've seen this in another clue).
'ran' placed around 'i' is 'rain'.
'rain'+'gauge'='RAIN-GAUGE'
(Other definitions for rain gauge that I've seen before include "one can measure water" , "met recorder?" , "Pluviometer" , "Meteorological instrument" , "The measure of a meteorologist" .)