With a trio of kings invading, ruin developing area (8)
I believe the answer is:
darkroom
'developing area' is the definition.
(I've seen this before)
'with a trio of kings invading ruin' is the wordplay.
'with' is an insertion indicator.
'a trio of' becomes 'ar' (I am not sure about this - if you are sure you should believe this answer much more).
'kings' becomes 'RK' (R means rex - 'king' in Latin - and K means king).
'invading' means one lot of letters goes inside another.
'ruin' becomes 'doom' (doom means ruin or disaster).
'ar' placed around 'rk' is 'arkr'.
'arkr' inserted within 'doom' is 'DARKROOM'.
Can you help me to learn more?
(Other definitions for darkroom that I've seen before include "In which to develop photographs" , "Film-developing cubicle" , "development area" , "developing area" , "Gloomy place where photos are processed" .)