Kind of pipe that has distributed H2O over reasonable area (6)
I believe the answer is:
hookah
'kind of pipe' is the definition.
'distributed h2o over reasonable area' is the wordplay.
'distributed hoo' becomes 'hoh' (I can't explain this - if you can you should believe this answer much more).
'over' is an insertion indicator.
'reasonable' becomes 'ok'.
'area' becomes 'a' (maths abbreviation).
'ok'+'a'='oka'
'hoh' going around 'oka' is 'HOOKAH'.
'that has' acts as a link.
(Other definitions for hookah that I've seen before include "Sort of pipe" , "Oriental tobacco pipe that cools the smoke" , "Water-cooled tobacco pipe" , "Pipe for smoking marijuana etc" , "Oriental water pipe for smoking" .)