Something bad about terriers being the wrong colour! (5)
I believe the answer is:
stain
'the wrong colour' is the definition.
'stain' can be an answer for 'colour' (I've seen this before). I am not sure about the 'the wrong' bit.
'something bad about terriers' is the wordplay.
'something bad' becomes 'sin' (something morally bad).
'about' is an insertion indicator.
'terriers' becomes 'ta' (nickname for Territorial Army members).
'sin' going around 'ta' is 'STAIN'.
'being' acts as a link.
Can you help me to learn more?
(Other definitions for stain that I've seen before include "Stigma" , "Unsightly mark" , "Discoloration" , "Colour, dye" , "Satin (anag.)" .)