Action by troops back to protect fortification (10)
I believe the answer is:
battlement
'fortification' is the definition.
(battlement is a kind of fortification)
'action by troops back to protect' is the wordplay.
'action' becomes 'battle' (military action).
'by troops' becomes 'men' (I've seen this in another clue. I am not sure about the 'by' bit.).
'back to' says to take the final letters.
The final letter of 'protect' is 't'.
'battle'+'men'+'t'='BATTLEMENT'
Can you help me to learn more?
(Other definitions for battlement that I've seen before include "fortification" , "parapet" , "Top of a fortified wall" , "Crenellation" .)