Army I'd assembled is countless (6)
I believe the answer is:
myriad
'countless' is the definition.
('myriad' can be a synonym of 'countless')
'army i'd assembled' is the wordplay.
'assembled' indicates an anagram.
'army'+'id'='armyid'
'armyid' anagrammed gives 'MYRIAD'.
'is' is the link.
(Other definitions for myriad that I've seen before include "Numberless" , "immense quantity" , "Countless - very large but indeterminate number" , "a vast number, perhaps innumerable" , "Countless, extremely large number" .)