Sign nothing in front of the work force (4)
I believe the answer is:
omen
'sign' is the definition.
(I've seen this before)
'nothing in front of the work force' is the wordplay.
'nothing' becomes 'o' (looks like zero - 0).
'in front of' is a charade indicator (letters next to each other).
'the work force' becomes 'men' ('men' can be a synonym of 'work force').
'o'+'men'='OMEN'
(Other definitions for omen that I've seen before include "Augury, portent" , "writing on the wall" , "Forewarning, harbinger" , "premonition" , "Augur" .)