A jar full of mainly creamy fruit (7)
I believe the answer is:
apricot
'fruit' is the definition.
(type of fruit)
'a jar full of mainly creamy' is the wordplay.
'jar' becomes 'pot' (I've seen this in other clues).
'full of' is an insertion indicator.
'mainly' means to remove the last letter.
'creamy' becomes 'rich' (as in rich or creamy food).
'rich' with its last letter taken off is 'ric'.
'pot' going around 'ric' is 'pricot'.
'a'+'pricot'='APRICOT'
(Other definitions for apricot that I've seen before include "A fruit-tree" , "Small, peach-like fruit" , "Asian fruit" , "Yellowish fruit like a small peach" , "Orange-coloured fruit" .)