Nothing about a tack (4)
I believe the answer is:
nail
'tack' is the definition.
(thesaurus)
'nothing about a' is the wordplay.
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'about' means one lot of letters goes inside another.
'nil' enclosing 'a' is 'NAIL'.
(Other definitions for nail that I've seen before include "Brad or tack" , "Fasten, catch (criminal)" , "Carpentry fastener" , "Hammered individual" , "Apprehend" .)