Take top off slightly open pot (3)
I believe the answer is:
jar
'pot' is the definition.
(thesaurus)
'take top off slightly open' is the wordplay.
'take top off' means to remove the first letter (I've seen 'taking top off' mean this).
'slightly open' becomes 'ajar' (as in a door left ajar).
'ajar' with its first letter taken away is 'JAR'.
(Other definitions for jar that I've seen before include "Cylindrical container as for jam" , "Glass of beer; clash" , "Container; be incongruous" , "Wide-mouthed container" , "Storage container" .)