Sir, some compounds are chemicals with same molecular weight but different arrangement of atoms (7)
I believe the answer is:
isomers
'chemicals with same molecular weight but different arrangement of atoms' is the definition.
I can't tell whether this defines the answer.
'sir some compounds' is the wordplay.
'compounds' is an anagram indicator.
'sir'+'some'='sirsome'
'sirsome' anagrammed gives 'ISOMERS'.
'are' acts as a link.
Can you help me to learn more?
(Other definitions for isomers that I've seen before include "Chemical compounds" , "Radicals" .)