A jar containing mostly mellow fruit (7)
I believe the answer is:
apricot
'fruit' is the definition.
(I know that apricot is a type of fruit)
'a jar containing mostly mellow' is the wordplay.
'jar' becomes 'pot' (I've seen this in other clues).
'containing' means one lot of letters goes inside another.
'mostly mellow' becomes 'ric' (I can't justify this - if you can you should give a lot more credence to this answer).
'pot' going around 'ric' is 'pricot'.
'a'+'pricot'='APRICOT'
Can you help me to learn more?
(Other definitions for apricot that I've seen before include "Asian fruit" , "Orange-yellow colour; fruit" , "Small, peach-like fruit" , "Orange-coloured fruit" , "Fruit from a tropic" .)