Soldiers at front of unit take seditious action (6)
I believe the answer is:
revolt
'take seditious action' is the definition.
(sedition is revolution or rebellion)
'soldiers at front of unit' is the wordplay.
'soldiers' becomes 'RE' (Royal Engineers).
'at front of' means one lot of letters go next to another.
'unit' becomes 'volt' (volt is a kind of unit).
're'+'volt'='REVOLT'
(Other definitions for revolt that I've seen before include "Rise in rebellion" , "Rebel, rise up against" , "Rebel; rebellion" , "Rising" , "Upset" .)