Army beginning to approach plant (5)
I believe the answer is:
hosta
'plant' is the definition.
The answer is an example.
'army beginning to approach' is the wordplay.
'army' becomes 'host' (host is a kind of army).
'beginning to' says to take the initial letters.
The first letter of 'approach' is 'a'.
'host'+'a'='HOSTA'
(Other definitions for hosta that I've seen before include "Garden plant prized for its foliage" , "flowering plant" , "Garden plant with ornamental foliage" , "found in a bed, perhaps" , "Ornamental foliage plant; as hot (anag.)" .)