It depends on cow's companion wed sent back (6)
I believe the answer is:
dewlap
'it depends on cow's' is the definition.
The definition suggests a singular noun which matches the answer.
'companion wed sent back' is the wordplay.
'companion' becomes 'pal' (I've seen this in another clue).
'sent back' says the letters should be written in reverse.
'pal'+'wed'='palwed'
'palwed' backwards is 'DEWLAP'.
Can you help me to learn more?
(Other definitions for dewlap that I've seen before include "Flap" , "Fold of loose skin that hangs beneath throat" , "Fold of flesh hanging below chin" , "Fold of skin hanging below the chin" , "Throat's loose skin" .)