On the way back, left the last calciferol vitamin as not wanted (2,4)
I believe the answer is:
de trop
'not wanted' is the definition.
(I've seen this before)
'on the way back left the last calciferol' is the wordplay.
'on the way back' shows that the letters should be reversed in order.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'last' suggests the final letters.
'calciferol' becomes 'D' (synonyms).
The last letter of 'the' is 'e'.
'port'+'e'+'d'='ported'
'ported' back-to-front is 'DE TROP'.
'vitamin as' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for de trop that I've seen before include "Deport from France, being in the way?" , "not to drink too much" , "Excessive - not wanted" , "Unwanted - excessive (French)" , "surplus to requirements" .)