Dump serving as nest for tiny bird ascending (6)
I believe the answer is:
peewit
'dump' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'serving as nest for tiny bird ascending' is the wordplay.
'serving as nest for' is an insertion indicator (some letters provide a home for others).
'tiny' becomes 'wee' (I've seen this before).
'bird' becomes 'tip' (I can't explain this - if you can you should believe this answer much more).
'ascending' shows that the letters should be reversed in order.
'wee' going into 'tip' is 'tiweep'.
'tiweep' written backwards gives 'PEEWIT'.
(Other definitions for peewit that I've seen before include "Northern lapwing" , "Does it weep, this bird?" , "Lapwing - ie wept (anag)" , "Lapwing turned up in a wee pit" , "Kind of plover; let it weep" .)