Makes dirty joke finally in second bar (6)
I believe the answer is:
smears
'makes dirty' is the definition.
(smearing is a kind of making dirty)
'joke finally in second bar' is the wordplay.
'finally' says to take the final letters.
'in' is an insertion indicator.
'second' becomes 's' (common abbreviation).
'bar' becomes 'mars' (chocolate bar).
The last letter of 'joke' is 'e'.
's'+'mars'='smars'
'e' placed inside 'smars' is 'SMEARS'.
(Other definitions for smears that I've seen before include "Spreads with dirty or greasy substance" , "may be examined in the lab." , "Black marks" , "Maligns" , "False attacks on someone's good name" .)