Poison bringing a temperature — turning yellow before long (8)
I believe the answer is:
atropine
'poison' is the definition.
(I know that atropine is a type of poison)
'a temperature turning yellow before long' is the wordplay.
'temperature' becomes 't' (abbreviation).
'turning' is a reversal indicator.
'yellow' becomes 'or' (heraldic term).
'before' means one lot of letters go next to another.
'long' becomes 'pine' (to pine for someone is to long for them).
'or' written backwards gives 'ro'.
'a'+'t'+'ro'+'pine'='ATROPINE'
'bringing' is the link.
(Other definitions for atropine that I've seen before include "Poisonous substance" , "Poison found in deadly nightshade used as a drug" , "A pointer (anag) - belladonna" , "Drug obtained from deadly nightshade" .)