Small display showing promise (5)
I believe the answer is:
swear
'promise' is the definition.
(I've seen this before)
'small display' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'display' becomes 'wear' (I've seen this before).
's'+'wear'='SWEAR'
'showing' is the link.
(Other definitions for swear that I've seen before include "Use bad language" , "Use indecent or blasphemous language" , "Utter obscenities or take an oath" , "Express annoyance" , "Avow" .)