To down tools can be a blow (6)
I believe the answer is:
strike
This clue is a double definition.
'to down tools' is the first definition.
(I've seen this before)
'a blow' is the second definition.
(strike is a kind of blow)
'can be' is the link.
(Other definitions for strike that I've seen before include "Form of industrial action" , "Industrial dispute - everybody out" , "Hit the labour withdrawal" , "Hit or take industrial action" , "Withdraw labour in industrial dispute" .)