No longer employ top cost inside as justification (6)
I believe the answer is:
excuse
'justification' is the definition.
(I know that excuse is a type of defence)
'no longer employ top cost inside' is the wordplay.
'no longer' becomes 'ex' (prefix meaning former or onetime).
'employ' becomes 'use' (synonyms).
'top cost' becomes 'c' (I am not sure about this - if you are sure you should believe this answer much more).
'inside' is an insertion indicator.
'ex'+'use'='exuse'
'exuse' placed around 'c' is 'EXCUSE'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for excuse that I've seen before include "Let off or make allowances for" , "Defence of offensive behaviour" , "Pardon, or release from obligation" , "Let off the hook" , "Grant exemption or release to" .)