Reader, if involved, is made less dense (8)
I believe the answer is:
'made less dense' is the definition.
I can't tell whether this defines the answer.
'reader if involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'readerif' is an anagram of 'RAREFIED'.
'is' is the link.
Can you help me to learn more?
(Other definitions for rarefied that I've seen before include "Air at high altitudes" , "(Of gas) at very low pressure" , "Of air, thin as on high mountain" , "Elevated in nature or style" , "'Of lower pressure than average, at high altitudes (8)'" .)