Last bits of boring seminar are exceptionally dull (4)
I believe the answer is:
grey
'dull' is the definition.
(I've seen this before)
'last bits of boring seminar are exceptionally' is the wordplay.
'last bits of' suggests the final letters.
The last letters of 'boring seminar are exceptionally' are 'GREY'.
(Other definitions for grey that I've seen before include "15 [JANE] the short-lived queen" , "Colour of ashes" , "(Of weather) dull, cloudy" , "Ashen" , "Colour of lead" .)