Dull relative? That is very impolite, ultimately (7)
I believe the answer is:
relieve
'dull' is the definition.
(thesaurus)
'relative? that is very impolite ultimately' is the wordplay.
'relative?' becomes 'rel' (mathematical abbreviation**).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'very' becomes 'v' (abbreviation).
'ultimately' says to take the final letters.
The final letter of 'impolite' is 'e'.
'rel'+'ie'+'v'+'e'='RELIEVE'
(Other definitions for relieve that I've seen before include "Alleviate pain or stress" , "Take a job over from - bring aid" , "Alleviate, assuage" , "Alleviate (eg, pain)" , "Set free" .)