Open tin sufficed (6)
I believe the answer is:
candid
'open' is the definition.
(I know that open can be written as candid)
'tin sufficed' is the wordplay.
'tin' becomes 'can' (synonyms).
'sufficed' becomes 'did' ('do' can be a synonym of 'suffice').
'can'+'did'='CANDID'
(Other definitions for candid that I've seen before include "Forthrightly sincere" , "Frank and outspoken" , "Plain" , "Frank, forthright" , "Honest and open about what one thinks" .)