Tense about a certain prize (8)
I believe the answer is:
treasure
'prize' is the definition.
(synonyms)
'tense about a certain' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'about' becomes 'rea' (I've seen this in other clues).
'a certain' becomes 'sure' (I've seen this before).
't'+'rea'+'sure'='TREASURE'
(Other definitions for treasure that I've seen before include "Hoarded riches" , "Classic adventure story" , "Gold on this island" , "... story by R.L. Stevenson" , "masterpiece?" .)