Endless words to acclaim a hollow rockery's little flower (9)
I believe the answer is:
tributary
'flower' is the definition.
(part of a river - something that flows)
'endless words to acclaim a hollow' is the wordplay.
'endless' means to remove the last letter.
'words' becomes 'tribute' (I have seen 'Words of approbation ' mean 'tribute' so perhaps 'words' could also mean 'tribute').
'to acclaim a hollow' becomes 'ary' (I can't explain this - if you can you should believe this answer much more).
'tribute' with its final letter taken away is 'tribut'.
'tribut'+'ary'='TRIBUTARY'
'rockery's little' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for tributary that I've seen before include "River running into another" , "River that flows into larger river or lake" , "River that feeds a larger one" , "River flowing into a larger one" , "River that flows into a larger one" .)