Old fossil, and sour I suspect (8)
I believe the answer is:
dinosaur
'old fossil' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'and sour i suspect' is the wordplay.
'suspect' indicates an anagram (a suspicious or unusual spelling).
'and'+'sour'+'i'='andsouri'
'andsouri' with letters rearranged gives 'DINOSAUR'.
Can you help me to learn more?
(Other definitions for dinosaur that I've seen before include "Enormous extinct reptile of ancient times" , "Extinct reptilian animals" , "Out of date person or thing" , "type unable to adapt" , "square figure!" .)