Justify making a definite start and finish (6)
I believe the answer is:
defend
'justify' is the definition.
(I've seen this before)
'a definite start and finish' is the wordplay.
'a definite start' becomes 'def' (I can't justify this - if you can you should believe this answer much more).
'and' means one lot of letters go next to another.
'finish' becomes 'end' (synonyms).
'def'+'end'='DEFEND'
'making' acts as a link.
Can you help me to learn more?
(Other definitions for defend that I've seen before include "Resist an attack" , "Protect - endorse" , "Help or protect against attack" , "Justify" , "Block, as in cricket" .)