Where army hopes to gain land beforehand (2,7)
I believe the answer is:
in advance
'beforehand' is the definition.
(synonyms)
'where army hopes to gain land' is the wordplay.
I cannot quite see how this works, but
'gain' could be 'advance' (synonyms) and 'advance' is present in the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for in advance that I've seen before include "Before the event" , "Beforehand - can invade accordingly (2.7)" , "Ahead in place or time" , "early" .)