Fruit left one extremely morose (4)
I believe the answer is:
lime
'fruit' is the definition.
(lime is a kind of fruit)
'left one extremely morose' is the wordplay.
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'extremely' means to remove the middle letters (letters on the extremes of the word are used).
'morose' with its middle taken out is 'me'.
'l'+'i'+'me'='LIME'
(Other definitions for lime that I've seen before include "Sharp-tasting fruit" , "Caustic calcium" , "Tree, linden or citrus" , "Having no money" , "Bright light green" .)