It's rare for this morsel to be cheese on toast! (3)
I believe the answer is:
bit
'this morsel to be cheese on toast' is the definition.
'bit' can be an answer for 'morsel' (synonyms). I'm unsure of the remainder of the definition.
'it's rare' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
The remaining letters 'bi' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'for' acts as a link.
Can you help me to learn more?
(Other definitions for bit that I've seen before include "Boring tip of a drill" , "Took effect - fragment" , "which is between one's teeth?" , "Snaffle - small piece" , "Part of 21 across" .)