US soldier acting strangely sees mammoth (8)
I believe the answer is:
gigantic
'mammoth' is the definition.
(I know that mammoth can be written as gigantic)
'us soldier acting strangely' is the wordplay.
'us soldier' becomes 'GI' (informal term for a US soldier).
'strangely' is an anagram indicator.
'acting' is an anagram of 'gantic'.
'gi'+'gantic'='GIGANTIC'
'sees' is the link.
(Other definitions for gigantic that I've seen before include "Sizeable" , "Hugh,colossal" , "Extremely large" , "walloping" , "Titanic" .)