Brave, secure up in tank (7)
I believe the answer is:
valiant
'brave' is the definition.
(similar in meaning)
'secure up in tank' is the wordplay.
'secure' becomes 'nail' (both can mean to apprehend or capture).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' is an insertion indicator.
'tank' becomes 'vat' (I've seen this before).
'nail' in reverse letter order is 'lian'.
'lian' inserted within 'vat' is 'VALIANT'.
(Other definitions for valiant that I've seen before include "Dauntless" , "Heroic" , "Brave - resolute" , "Stout" , "Brave in battle" .)