Over where a rat's moving, fellow encounters unknown bird (4,10)
I believe the answer is:
manx shearwater
'bird' is the definition.
(Manx shearwater is an example)
'over where a rat's moving fellow encounters unknown' is the wordplay.
'over' says to put letters next to each other.
'moving' indicates anagramming the letters.
'fellow encounters unknown' becomes 'manx' (I can't explain this - if you can you should believe this answer much more).
'where'+'a'+'rats'='wherearats'
'wherearats' with letters rearranged gives 'shearwater'.
'shearwater' after 'manx' is 'MANX SHEARWATER'.
Can you help me to learn more?
(Other definitions for manx shearwater that I've seen before include "Dark-backed seabird, Puffinus puffinus" , "European seabird" , "Bird" .)