House god reaching very advanced stage of development (5)
I believe the answer is:
larva
'stage of development' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'house god reaching very advanced' is the wordplay.
'house god' becomes 'lar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'reaching' means one lot of letters go next to another.
'very' becomes 'v' (abbreviation).
'advanced' becomes 'a'.
'lar'+'v'+'a'='LARVA'
Can you help me to learn more?
(Other definitions for larva that I've seen before include "Immature form of caterpillar" , "Immature life form" , "a developmental stage" , "E.g. caterpillar form" , "Woodworm" .)