Appear again, as army corps heading north to meet another southbound (2-6)

'appear again' is the definition.
('re-emerge' can be a synonym of 'appear again')

'army corps heading north to meet another southbound' is the wordplay.
'army corps' becomes 'reme' (Royal Electrical and Mechanical Engineers).
'heading north' says the letters should be written in reverse.
'to meet' is a charade indicator (letters next to each other).
'another' becomes 're' (I've seen this in other clues).
'southbound' becomes 'ge' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'reme' reversed gives 'emer'.
'emer' after 're' is 'reemer'.
'reemer'+'ge'='RE-EMERGE'

'as' acts as a link.