Strength outsiders have displayed (5)
I believe the answer is:
shown
'displayed' is the definition.
(I've seen this in another clue)
'strength outsiders have' is the wordplay.
'outsiders' means to remove the middle letters (only the letters on the outside of the word remain).
'have' becomes 'own' (to have something is to own it).
'strength' with its centre taken out is 'sh'.
'sh'+'own'='SHOWN'
(Other definitions for shown that I've seen before include "Exhibited" , "Displayed" , "Explained" , "Demonstrated" .)