Reader, if involved, is made less dense (8)
I believe the answer is:
rarefied
'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).
'reader'+'if'='readerif'
'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)'" .)