Justify five being on show (9)
I believe the answer is:
vindicate
'justify' is the definition.
(I know that justify can be written as vindicate)
'five being on show' is the wordplay.
'five' becomes 'V' (Roman numeral).
'being on' says to put letters next to each other.
'show' becomes 'indicate' (synonyms).
'v'+'indicate'='VINDICATE'
(Other definitions for vindicate that I've seen before include "Avenge or exonerate" , "Justify by evidence or results" , "Deflect criticism" , "Make good a claim" , "Successfully defend" .)