Plot by Jack introducing primarily unjustified smear (6)
I believe the answer is:
bedaub
'smear' is the definition.
(synonyms)
'plot by jack introducing primarily unjustified' is the wordplay.
'plot' becomes 'bed' (a plot of land).
'by' means one lot of letters go next to another.
'jack' becomes 'ab' (both can mean a sailor).
'introducing' is an insertion indicator (some letters introduced or inserted into others).
'primarily' says to take the initial letters.
The initial letter of 'unjustified' is 'u'.
'ab' enclosing 'u' is 'aub'.
'bed'+'aub'='BEDAUB'
(Other definitions for bedaub that I've seen before include "Smear (with a sticky substance)" , "Crudely paint" , "Smear with something sticky" , "Splodge with paint" .)