Guarantee a number finish with a painful expression (6)
I believe the answer is:
avouch
'guarantee a number finish' is the definition.
'avouch' can be an answer for 'guarantee' (I've seen this before). I'm unsure of the 'a number finish' bit.
'a painful expression' is the wordplay.
'a painful' becomes 'av' (I can't justify this - if you can you should give a lot more credence to this answer).
'expression' becomes 'ouch' (I have seen 'Expression of pain' mean 'ouch' so perhaps 'expression' could also mean 'ouch').
'av'+'ouch'='AVOUCH'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for avouch that I've seen before include "Assert (archaic)" , "Assert; guarantee" .)