After cautious start, I back a tank (7)
I believe the answer is:
cistern
'a tank' is the definition.
(cistern is a kind of tank)
'after cautious start i back' is the wordplay.
'after' means one lot of letters go next to another.
'start' indicates taking the first letters.
'back' becomes 'stern' (stern means the back of a boat).
The initial letter of 'cautious' is 'c'.
'c' next to 'i' is 'ci'.
'ci'+'stern'='CISTERN'
(Other definitions for cistern that I've seen before include "Toilet tank [ital]" , "Vat" , "bit of toilet" , "Water tank for toilet" , "Water storage tank" .)