Briefly look after certain valuables (8)
I believe the answer is:
treasure
'valuables' is the definition.
(I've seen this before)
'briefly look after certain' is the wordplay.
'briefly' means to remove the last letter.
'look after' becomes 'treat' (as in a nurse treating patients).
'certain' becomes 'sure' (synonyms).
'treat' with its final letter taken away is 'trea'.
'trea'+'sure'='TREASURE'
(Other definitions for treasure that I've seen before include "Value highly" , "Stored-up wealth" , "Book by Stevenson" , "Highly valued possession" , "Highly-prized possession" .)