Ultimately, this display shows promise (5)
I believe the answer is:
swear
'promise' is the definition.
(as in swearing something is true)
'ultimately this display' is the wordplay.
'ultimately' suggests the final letters.
'display' becomes 'wear' (to wear something is to display it on one's person).
The final letter of 'this' is 's'.
's'+'wear'='SWEAR'
'shows' acts as a link.
(Other definitions for swear that I've seen before include "Eff and blind" , "Curse" , "Affirm" , "Use bad language" , "Avow" .)