Some bighead ultimately developed (5)
I believe the answer is:
adult
'developed' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'some bighead ultimately' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'ADULT' can be found hidden inside 'bighead ultimately'.
Can you help me to learn more?
(Other definitions for adult that I've seen before include "used to describe dirty movies" , "No child" , "(Of an animal) fully developed" , "Full-grown" , "Older person" .)