Deduce there's justification (6)
I believe the answer is:
reason
I believe this clue is a double definition.
'deduce' is the first definition.
(I've seen this before)
'justification' is the second definition.
(I've seen this before)
'there's' is the link.
(Other definitions for reason that I've seen before include "The right exercise of the mind" , "Justification or motive" , "Explanation or justification" , "Cause for sanity" , "use of wit" .)