It's a singular cereal! (3)
I believe the answer is:
oat
'singular cereal' is the definition.
(I've seen this before)
'it's a' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
A single letter 'o' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oat that I've seen before include "Cereal grass" , "Cereal (singular)" , "Basis of porridge" , "Cereal plant" , "Food grain" .)