Brownish line in split peas, possibly (5)
I believe the answer is:
sepia
'brownish' is the definition.
(I've seen this before)
'line in split peas possibly' is the wordplay.
'line in' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'split' is an insertion indicator (some letters split a word into two).
'possibly' indicates an anagram.
'peas' anagrammed gives 'sepa'.
'i' put within 'sepa' is 'SEPIA'.
Can you help me to learn more?
(Other definitions for sepia that I've seen before include "Brown colour of old photo" , "Old drawing" , "Rich reddish-brown colour" , "source of ink" , "Former photographic pigment" .)