Barrier on waterway under pressure provides protection from water (4,6)
I believe the answer is:
damp course
'protection from water' is the definition.
(I've seen this before)
'barrier on waterway under pressure' is the wordplay.
'barrier' becomes 'dam' (dam is a kind of barrier).
'on' says to put letters next to each other.
'waterway under' becomes 'course' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pressure' becomes 'p' (abbreviation).
'course' after 'p' is 'pcourse'.
'dam'+'pcourse'='DAMP-COURSE'
'provides' is the link.
Can you help me to learn more?
(Other definitions for damp course that I've seen before include "housing benefit?" , "waterproofing" , "Protection from water" , "It'll fortify against bad weather" , "protective component of building" .)