Greyish-brown in colour, insect is unwanted (9)
I believe the answer is:
redundant
'unwanted' is the definition.
(I've seen this before)
'greyish-brown in colour insect' is the wordplay.
'greyish-brown' becomes 'dun' (I've seen this before).
'in' means one lot of letters goes inside another.
'colour' becomes 'red' (red is a kind of colour).
'insect' becomes 'ant' (ant is a kind of insect).
'dun' put into 'red' is 'redund'.
'redund'+'ant'='REDUNDANT'
'is' is the link.
(Other definitions for redundant that I've seen before include "unwanted" , "out of a job" , "no longer needed" , "Superfluous to requirements" , "jobless!" .)