Quite strict in, thus, returning water-storage vessels (8)
I believe the answer is:
cisterns
'vessels' is the definition.
(cistern is a kind of vessel)
'quite strict in thus returning' is the wordplay.
'quite strict' becomes 'stern' (synonyms. I am not sure about the 'quite' bit.).
'in' is an insertion indicator.
'thus' becomes 'sic' ('thus' in Latin).
'returning' shows that the letters should be reversed in order.
'sic' written backwards gives 'cis'.
'stern' going inside 'cis' is 'CISTERNS'.
'water-storage' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Another definition for cisterns that I've seen is " Water tanks".)