Bird covered in urine? It's wee (6)
I believe the answer is:
petite
'it's wee' is the definition.
(wee can mean small)
'bird covered in urine?' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'covered in' means one lot of letters goes inside another.
'urine?' becomes 'pee' (pee can informally mean urine).
'tit' inserted inside 'pee' is 'PETITE'.
(Other definitions for petite that I've seen before include "Wee" , "Small, slim (girl)" , "Attractively small" , "Small and dainty from France" , "Short" .)