Inflammation young relative talked of by brook (7)
I believe the answer is:
sunburn
'inflammation' is the definition.
(sunburn is a type of inflammation)
'young relative talked of by brook' is the wordplay.
'young relative' becomes 'son' (a child).
'talked of' indicates a 'sounds like' (homophone) clue.
'by' means one lot of letters go next to another.
'brook' becomes 'burn' (I've seen this before).
'son' is a homophone of 'sun'.
'sun'+'burn'='SUNBURN'
(Other definitions for sunburn that I've seen before include "Tan (or unsuccessful attempt at it)" , "Solar skin damage" , "Painful browning of the skin" , "Overexposure to UV" , "Effect of too much UV" .)