Tanks being clear sign to involve leader of military (9)
I believe the answer is:
'tanks being clear' is the definition.
(clear tanks for displaying fish)
'sign to involve leader of military' is the wordplay.
'sign' becomes 'aquarius' (Aquarius is an example).
'to involve' means one lot of letters goes inside another.
'leader of' indicates taking the first letters.
The initial letter of 'military' is 'm'.
'aquarius' enclosing 'm' is 'AQUARIUMS'.
(Another definition for aquariums that I've seen is " Tanks or bowls for fish".)