Wanders over, entering site of battle (5)
I believe the answer is:
moons
'wanders' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how one could define the other.
'entering site of battle' is the wordplay.
'entering' means one lot of letters goes inside another.
'site of' becomes 'o' (I can't justify this - if you can you should give a lot more credence to this answer).
'battle' becomes 'mons' (Battle of Mons, WWo battle).
'o' inserted inside 'mons' is 'MOONS'.
'over' acts as a link.
Can you help me to learn more?
(Other definitions for moons that I've seen before include "months" , "Satellites of planets" , "carry on" , "Natural satellites" , "Orbiting bodies" .)