Lust! It's wasted on the extremely fussy, dull, ... (8)
I believe the answer is:
stultify
'dull' is the definition.
(I've seen this before)
'lust it's wasted on the extremely fussy' is the wordplay.
'wasted' is an anagram indicator.
'on' means one lot of letters go next to another.
'the extremely' means to remove the middle letters (only the extremes of the word are used).
'fussy' with its centre taken out is 'fy'.
'lust'+'it'='lustit'
'lustit' is an anagram of 'stulti'.
'stulti'+'fy'='STULTIFY'
(Other definitions for stultify that I've seen before include "Dull the mind with tedious tasks" , "Cause to feel bored" , "Render ineffectual - lusty fit (anag)" , "Make useless" , "Cause to appear stupid" .)