Sweet substance needed for upstart in rags, possibly (5)
I believe the answer is:
sugar
'sweet substance' is the definition.
(I've seen this before)
'upstart in rags possibly' is the wordplay.
'upstart' becomes 'u' (start of the word 'up').
'in' means one lot of letters goes inside another.
'possibly' indicates an anagram.
'rags' with letters rearranged gives 'sgar'.
'u' going into 'sgar' is 'SUGAR'.
'needed for' acts as a link.
(Other definitions for sugar that I've seen before include "flattery" , "Damn" , "White crystalline carbohydrate" , "Make more palatable" , "Sucrose" .)