A pot is slightly open (4)
I believe the answer is:
ajar
'slightly open' is the definition.
(as in a door left ajar)
'a pot' is the wordplay.
'pot' becomes 'jar' ().
'a'+'jar'='AJAR'
'is' acts as a link.
(Other definitions for ajar that I've seen before include "A little bit open, like door" , "near closing?" , "Barely open" , "Partly open" , "Half-closed" .)