Ludicrous rules essentially a huge lab reforms (9)
I believe the answer is:
laughable
'ludicrous' is the definition.
(I've seen this before)
'rules essentially a huge lab reforms' is the wordplay.
'essentially' says to take the centre.
'reforms' indicates anagramming the letters.
The middle letter of 'rules' is 'l'.
'huge'+'lab'='hugelab'
'hugelab' with letters rearranged gives 'ughable'.
'l'+'a'+'ughable'='LAUGHABLE'
(Other definitions for laughable that I've seen before include "comic" , "Risible, ridiculous" , "Ridiculous, derisory" , "Incongruous, inviting ridicule" , "Ludicrous" .)