Waste from gulls usually, also numerous other sources (5)
I believe the answer is:
guano
'waste' is the definition.
(guano is a kind of waste)
'gulls usually also numerous other sources' is the wordplay.
'sources' says to take the initial letters.
The initial letters of 'gulls usually also numerous other' is 'GUANO'.
'from' acts as a link.
(Other definitions for guano that I've seen before include "dried excrement" , "One or more dropping by the coast" , "Seabird excrement, once prized as fertiliser" , "Sea bird droppings" , "Fertiliser from sea birds" .)