What a seaborne invasion will do for the army? (9)
I believe the answer is:
landforce
'what a seaborne invasion' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'will do for the army?' is the wordplay.
'will do' becomes 'land' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' is a charade indicator (letters next to each other).
'the army?' becomes 'force' (army is a kind of force).
'land'+'force'='LANDFORCE'
Can you help me to learn more?
(Another definition for landforce that I've seen is " Military unit".)