For six grand I'll finish off the night watch (5)
I believe the answer is:
vigil
'the night watch' is the definition.
(I've seen this before)
'for six grand i'll finish off' is the wordplay.
'for six' becomes 'vi' (I can't justify this - if you can you should give a lot more credence to this answer).
'grand' becomes 'g' (G is short for grand as in a thousand).
'finish off' means to remove the last letter.
'ill' with its last letter removed is 'il'.
'vi'+'g'+'il'='VIGIL'
Can you help me to learn more?
(Other definitions for vigil that I've seen before include "Watchful night" , "observance" , "Purposeful surveillance to guard or observe" , "Night service; watch" , "Night-time watch" .)