Agree to keep old tank mainly for one of our relatives (10)
I believe the answer is:
chimpanzee
'one of our relatives' is the definition.
(chimpanzees are related to humans)
'agree to keep old tank mainly' is the wordplay.
'agree' becomes 'chime'.
'to keep' is an insertion indicator.
'old tank' becomes 'panzer' (German World War Two tank).
'mainly' means to remove the last letter.
'panzer' with its final letter removed is 'panze'.
'chime' enclosing 'panze' is 'CHIMPANZEE'.
'for' is the link.
(Other definitions for chimpanzee that I've seen before include "Anthropoid ape" , "Primate" , "animal" , "Close relative of man" , "Great ape" .)