Short rhyme upheld by Colet (7)
I believe the answer is:
couplet
'short' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'rhyme upheld by colet' is the wordplay.
'rhyme' becomes 'up' (I can't explain this - if you can you should give a lot more credence to this answer).
'upheld by' indicates putting letters inside.
'up' put into 'colet' is 'COUPLET'.
Can you help me to learn more?
(Other definitions for couplet that I've seen before include "piece of poetry" , "bit of poetry" , "Pair of verse lines" , "Pair of lines in poetry" , "Two lines of verse, often rhyming" .)