Leaders of social workers are threatening strike (4)
I believe the answer is:
swat
'strike' is the definition.
(thesaurus)
'leaders of social workers are threatening' is the wordplay.
'leaders of' indicates taking the first letters.
The initial letters of 'social workers are threatening' is 'SWAT'.
(Other definitions for swat that I've seen before include ".`Twas quite a blow" , "Hit and crush (a fly)" , "Swift, sharp blow" , "Twas something of a blow" , "Hit hard, say fly, with something flat" .)