Nothing fitted round a collar (4)
I believe the answer is:
nail
'collar' is the definition.
(I know that collar can be written as nail)
'nothing fitted round a' is the wordplay.
'nothing' becomes 'nil' (eg nil is zero in football).
'fitted round' means one lot of letters goes inside another.
'nil' placed around 'a' is 'NAIL'.
(Other definitions for nail that I've seen before include "Fixer (on finger?)" , "Covering on tip of the finger" , "Fixing spike" , "Expose as a lie" , "Horny covering of digit tip" .)