One fellow abandons dinosaur droppings (5)
I believe the answer is:
guano
'droppings' is the definition.
(term for bird droppings)
'one fellow abandons dinosaur' is the wordplay.
I cannot really understand how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
This explanation may well be incorrect...
(Other definitions for guano that I've seen before include "Excrement of sea birds, used as fertilizer" , "Dung of sea birds" , "Manure from birds - artificial fertiliser from fish" , "business to aid growth" , "Fertiliser from seabirds" .)