Turn up for yoga class, did you say? Boo and hiss! (12)
I believe the answer is:
onomatopoeia
'turn up for yoga' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'class did you say? boo and hiss' is the wordplay.
I cannot really see how this works, but
'say?' could be 'top' and 'top' is found in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for onomatopoeia that I've seen before include "Use of words conveying the sound of their meaning" , "Use of words sounding like what they name" , "form of imitation?" , "Use of words sounding as what they describe" , "a term that sounds right" .)