Nothing to secure a tack (4)
I believe the answer is:
nail
'tack' is the definition.
(thesaurus)
'nothing to secure a' is the wordplay.
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'to secure' is an insertion indicator.
'nil' enclosing 'a' is 'NAIL'.
(Other definitions for nail that I've seen before include "Finger feature (and tooth?)" , "Hard plate at end of digit" , "Hit this on the head to be dead right" , "for the carpenter?" , "Claw" .)