Collection of data, say, is dumped in River Test (8)
I believe the answer is:
registry
'collection' is the definition.
(I know that registry is a type of catalogue)
'say is dumped in river test' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'dumped in' is an insertion indicator.
'river' becomes 'r' (abbreviation).
'test' becomes 'try' (synonyms).
'eg'+'is'='egis'
'r'+'try'='rtry'
'egis' going inside 'rtry' is 'REGISTRY'.
'of data' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for registry that I've seen before include "Place where recorded list is kept" , "Place where written documents or records are kept" , "Office where records are kept" , "Official written record of names, events etc" , "where files may be observed" .)