Nasty accusation given attention by NCO (5)
I believe the answer is:
smear
'nasty accusation' is the definition.
'smear' can be an answer for 'accusation' (I have seen 'False accusation ' mean 'smear' so perhaps 'accusation' could also mean 'smear'). I'm not certain of the 'nasty' bit.
'attention by nco' is the wordplay.
'attention' becomes 'ear' (having someone's ear is having their attention).
'by' means one lot of letters go next to another.
'nco' becomes 'sm' (abbreviation for sergeant major, a type of NCO).
'ear' after 'sm' is 'SMEAR'.
'given' acts as a link.
Can you help me to learn more?
(Other definitions for smear that I've seen before include "Unwarranted accusation" , "False accusation; blur" , "False report" , "Dirty mark; a libel" , "Greasy/sticky mark" .)