Stain entered beneath the first of eight o's (5)
I believe the answer is:
eosin
'stain' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?
'entered beneath the first of eight o's' is the wordplay.
'entered' becomes 'in' (I am not sure about this - if you are sure you should believe this answer much more).
'beneath' means one lot of letters go next to another (in a down clue, some letters go below others).
'the first of eight' becomes 'e' (ist letter of 'eight').
'e'+'os'='eos'
'in' put after 'eos' is 'EOSIN'.
Can you help me to learn more?
(Other definitions for eosin that I've seen before include "Red dye" , "Red dyestuff" .)