23 across
Finish
mailing
out
17
(6)
Finish mailing out 17 (6)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
Finish is the definition. Take mailing, which gives SENDING, then remove the S — that S being "son", the answer to 17 down (SON) — leaving ENDING.