Army abandoning base to seize a treasure (5)
I believe the answer is:
hoard
'treasure' is the definition.
(I've seen this before)
'army abandoning base to seize a' is the wordplay.
'army' becomes 'horde' (horde means an army or force).
'abandoning base' means to remove the last letter (I've seen 'abandons base' mean this (in a down clue, base can mean the bottom letter)).
'to seize' means one lot of letters goes inside another.
'horde' with its final letter taken away is 'hord'.
'hord' placed around 'a' is 'HOARD'.
(Other definitions for hoard that I've seen before include "Amass secretly" , "Accumulate" , "Collection" , "accumulated wealth" , "A lot of people" .)