Reddish-brown colour one's seen in mushy peas (5)
I believe the answer is:
sepia
'reddish-brown colour' is the definition.
(I've seen this before)
'one's seen in mushy peas' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
an anagram of 'peas' is 'sepa' which is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sepia that I've seen before include "Brown of old photos" , "Brown with a red tinge" , "Pigment got from cuttlefish ink" , "Rich reddish-brown colour" , "Colourant" .)