Open tin was sufficient (6)
I believe the answer is:
candid
'open' is the definition.
(I know that open can be written as candid)
'tin was sufficient' is the wordplay.
'tin' becomes 'can' (synonyms).
'was sufficient' becomes 'did' (I am not sure about this - if you are sure you should believe this answer much more).
'can'+'did'='CANDID'
Can you help me to learn more?
(Other definitions for candid that I've seen before include "Truthful, frank" , "Frank - and CID (anag)" , "Very clear" , "Free and frank" , "Impartial" .)