Large mammal in river -- or bird passing over one (7)
I believe the answer is:
rorqual
'large mammal' is the definition.
'rorqual' can be an answer for 'mammal' (rorqual is a kind of mammal). I'm not certain of the 'large' bit.
'river or bird passing over one' is the wordplay.
I cannot really understand how this works, but
'river' could be 'r' (abbreviation for river) and 'r' is found in the answer.
'or' is within the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for rorqual that I've seen before include "Great swimmer" , "Whale with dorsal fin" , "Toothless creature" , "Finback whale" , "whalers' target" .)