Fantasise about breaching water barrier (5)
I believe the answer is:
dream
'fantasise' is the definition.
(I've seen this before)
'about breaching water barrier' is the wordplay.
'about' is an insertion indicator.
'breaching' becomes 're' (I can't explain this - if you can you should believe this answer much more).
'water barrier' becomes 'dam' (I've seen this in another clue).
're' inserted into 'dam' is 'DREAM'.
Can you help me to learn more?
(Other definitions for dream that I've seen before include "Distant ambition" , "Somniate" , "dozy state" , "person to good to be true" , "Bedroom vision?" .)