Tessa looked up something valuable (5)
I believe the answer is:
asset
'something valuable' is the definition.
(I've seen this in another clue)
'tessa looked up' is the wordplay.
'looked up' says the letters should be written in reverse (I've seen 'looking up' mean this (in a down clue, letters go up)).
'tessa' written backwards gives 'ASSET'.
(Other definitions for asset that I've seen before include "One bringing benefit" , "Item well worth having" , "Valuable thing" , "blessing" , "A thing of value" .)