US soldier, with pound, gets hold of small creatures and a big one (5,5)

'a big one' is the definition.
(large marine creature)

'us soldier with pound gets hold of small creatures' is the wordplay.
'us soldier' becomes 'GI' (informal term for a US soldier).
'with' says to put letters next to each other.
'pound' becomes 'quid' (UK slang for a pound).
'gets hold of' means one lot of letters goes inside another.
'small creatures' becomes 'ants' (ants are insects).
'gi'+'quid'='giquid'
'giquid' going around 'ants' is 'GIANT SQUID'.

'and' is the link.