Hammer and nails initially in sack (4)
I believe the answer is:
bang
'hammer' is the definition.
(thesaurus)
'nails initially in sack' is the wordplay.
'initially' says to take the initial letters.
'in' is an insertion indicator.
'sack' becomes 'bag' (I've seen this in another clue).
The initial letter of 'nails' is 'n'.
'n' going inside 'bag' is 'BANG'.
'and' acts as a link.
(Other definitions for bang that I've seen before include "Slam" , "Sudden loud noise (often 'big')" , "Explosion; hair fringe" , "Vigorous blow" , "Report - something the hairdresser did?" .)