Brown animals heading west across island (5)
I believe the answer is:
sepia
'brown' is the definition.
(sepia is a kind of brown)
'animals heading west across island' is the wordplay.
'animals' becomes 'apes' (ape is a kind of animal**).
'heading west' says the letters should be written backwards.
'across' means one lot of letters goes inside another.
'island' becomes 'i'.
'apes' back-to-front is 'sepa'.
'sepa' placed around 'i' is 'SEPIA'.
(Other definitions for sepia that I've seen before include "Rich reddish-brown colour" , "Former photographic pigment" , "source of ink" , "Reddish-brown colour - easy as pie" , "Brown tint" .)