Physical ruin of reservoir wall with time (6)
I believe the answer is:
damage
'physical ruin' is the definition.
(physical ruin or deterioration)
'reservoir wall with time' is the wordplay.
'reservoir wall' becomes 'dam' (barrier in a reservoir).
'with' says to put letters next to each other.
'time' becomes 'age' (an age is a period of time).
'dam'+'age'='DAMAGE'
'of' acts as a link.
(Other definitions for damage that I've seen before include "Impairment, injury" , "Deface, harm" , "Cost (slang)" , "Harm, impair" , "Physical harm" .)