Fertiliser from birds (5)
I believe the answer is:
guano
'fertiliser from birds' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for guano that I've seen before include "Sea bird droppings" , "mess about by the sea" , "Manure from sea birds" , "Fertiliser from sea birds" , "One or more dropping by the coast" .)