Revolutionary soldiers' eastern line protects bridgehead (5)
I believe the answer is:
rebel
'revolutionary' is the definition.
(rebel is a kind of revolutionary)
'soldiers' eastern line protects bridgehead' is the wordplay.
'soldiers'' becomes 'rel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'eastern' becomes 'e'.
'line' means one lot of letters goes inside another.
'protects' indicates putting letters inside (outer letters protect inside letters).
'bridgehead' becomes 'b' (head letter of 'bridge').
'rel' placed around 'e' is 'reel'.
'reel' placed around 'b' is 'REBEL'.
(Other definitions for rebel that I've seen before include "Anti-establishment figure" , "Revolting person" , "Rejector of authority" , "One refusing to conform" , "(One who) revolt(s)" .)