When army leader seizes one big piece of ground! (4)
I believe the answer is:
asia
'ground' is the definition.
(Asia is an example)
'when army leader' is the wordplay.
'when' becomes 'as'.
'army leader' becomes 'ia' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'as'+'ia'='ASIA'
'seizes one big piece of' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for asia that I've seen before include "Continent in which India is situated" , "Large area" , "A is A (anag)" , "its pampered jades couldn't draw much?" , "Continent bordering three oceans" .)