“Open a tin” sufficed (6)
I believe the answer is:
candid
'open' is the definition.
(thesaurus)
'a tin sufficed' is the wordplay.
'a tin' becomes 'can' ('can' can be a synonym of 'tin').
'sufficed' becomes 'did' ('do' can be a synonym of 'suffice').
'can'+'did'='CANDID'
(Other definitions for candid that I've seen before include "Frank, honest" , "Truthful, frank" , "Truthful and straightforward" , "Frank and outspoken" , "Frank - unbiased" .)