Last of citrus peel left over (5)
I believe the answer is:
spare
'left over' is the definition.
(going spare)
'last of citrus peel' is the wordplay.
'last of' suggests the final letters.
'peel' becomes 'pare' (synonyms).
The final letter of 'citrus' is 's'.
's'+'pare'='SPARE'
(Other definitions for spare that I've seen before include "This tyre around the middle is fat" , "In reserve; pardon" , "Extra item just in case" , "Excess to requirements" , "Thin; elegantly simple" .)