Hybrid vigour evenly distributed in sheep, they're of such impressive size originally (9)
I believe the answer is:
heterosis
'hybrid vigour' is the definition.
Both the answer and definition are singular nouns.
Maybe there's a link between them I don't understand?
'evenly distributed in sheep they're of such impressive size originally' is the wordplay.
'evenly distributed in' means one should take alternating letters.
'originally' indicates taking the first letters.
The initial letters of 'of such impressive size' is 'osis'.
'sheep'+'theyre'='sheeptheyre'
The alternating letters of 'sheeptheyre' are 'heter'.
'heter'+'osis'='HETEROSIS'
Can you help me to learn more?
(Another definition for heterosis that I've seen is " Figure of speech".)