Giving away nothing but obsolete vessel (7)
I believe the answer is:
deadpan
'giving away nothing' is the definition.
'obsolete vessel' is the wordplay.
'obsolete' becomes 'dead' ('dead' can be similar in meaning to 'obsolete').
'vessel' becomes 'pan' (I have seen 'Shallow vessel ' mean 'pan' so perhaps 'vessel' could also mean 'pan').
'dead'+'pan'='DEADPAN'
'but' is the link.
(Other definitions for deadpan that I've seen before include "Showing no expression" , "without a flicker" , "With expressionless face" , "Lifeless" , "Betraying no emotion" .)