Injury due to how old the water barrier is (6)
I believe the answer is:
damage
'injury' is the definition.
(I know that damage is a type of injury)
'how old the water barrier is' is the wordplay.
I cannot quite understand how this works, but
'barrier' could be 'dam' (dam is a kind of barrier) and 'dam' is located in the answer.
'how old' could be 'age' (I've seen this before) and 'age' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'due to' is the link.
Can you help me to learn more?
(Other definitions for damage that I've seen before include "'Deface, impair (6)'" , "destruction" , "Impair, hurt" , "Ravages" , "Harm, impair" .)