It can be markedly unsightly (5)
I believe the answer is:
stain
'it can be markedly unsightly' is the definition.
The definition suggests a singular noun which matches the answer.
'it' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
The remaining letters 'sain' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stain that I've seen before include "Blemish; colour (wood)" , "Discoloration" , "Discolouration" , "Dirty mark; dye" , "Smear" .)