When the aged are upset about the wet weather, this is a useful system (8)
I believe the answer is:
drainage
'system' is the definition.
(drainage system is a kind of system)
'when the aged are upset about' is the wordplay.
'when' becomes 'rain' (I am not sure about this - if you are sure you should believe this answer much more).
'are upset' indicates an anagram.
'about' is an insertion indicator.
'aged' anagrammed gives 'dage'.
'rain' placed within 'dage' is 'DRAINAGE'.
'the wet weather this is a useful' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for drainage that I've seen before include "Removal of unwanted liquid" , "Drawing off liquid" , "Removal of waste water" , "System of waste pipes" , "Removal of surplus water" .)