Strike that has union members protesting, initially (5)
I believe the answer is:
thump
'strike' is the definition.
(thesaurus)
'that has union members protesting initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'that has union members protesting' is 'THUMP'.
(Other definitions for thump that I've seen before include "Dull heavy blow" , "Punch" , "Heavy blow with the hand" , "(Sound of) heavy blow" , "Hit hard" .)