Bird's tail replaced by companion in Yellowstone ravine (5)
I believe the answer is:
gulch
'companion in yellowstone ravine' is the definition.
'gulch' can be an answer for 'ravine' (gulch is a kind of ravine). I'm not certain of the 'companion in yellowstone' bit.
'bird's tail replaced' is the wordplay.
'bird' becomes 'gull' (gull is a kind of bird).
'tail' means to remove the last letter (I've seen 'tailed' mean this (to tail something can mean to remove its end)).
'replaced' becomes 'ch' (I can't justify this - if you can you should give a lot more credence to this answer).
'gull' with its final letter taken away is 'gul'.
'gul'+'ch'='GULCH'
'by' acts as a link.
Can you help me to learn more?
(Other definitions for gulch that I've seen before include "American ravine" , "Ravine - canyon" , "Narrow ravine (US)" , "valley" , "Narrow gorge with a stream in North America" .)