Fight soldiers by top of the parapet (10)
I believe the answer is:
battlement
'the parapet' is the definition.
(I've seen this before)
'fight soldiers by top of' is the wordplay.
'fight' becomes 'battle' (synonyms).
'soldiers' becomes 'men' (men can mean a group of soldiers).
'by' becomes 'at'.
'top of' means to remove the first letter (I've seen 'topped' mean this (to top something is to remove its top)).
'at' with its first letter taken off is 't'.
'battle'+'men'+'t'='BATTLEMENT'
(Other definitions for battlement that I've seen before include "parapet" , "Crenellation" , "fortification" , "Top of a fortified wall" .)