Fish kept in catchment areas (6)
I believe the answer is:
basins
'catchment areas' is the definition.
'basins' can be an answer for 'areas' (basin is a kind of area). I'm not certain of the 'catchment' bit.
'fish kept in' is the wordplay.
'fish' becomes 'bass' (bass is a kind of fish).
'kept' is an insertion indicator.
'bass' going around 'in' is 'BASINS'.
Can you help me to learn more?
(Other definitions for basins that I've seen before include "Bowl-shaped vessels" , "Sinks" .)