Foolish – as in, inefficient (in earliest stages) (7)
I believe the answer is:
asinine
'foolish' is the definition.
(similar in meaning)
'as in inefficient in earliest stages' is the wordplay.
I cannot really understand how this works, but
'as' could be 'in' (I've seen this in other clues) and 'in' is found within the answer.
'in' is within the answer.
The remaining letters 'ase' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for asinine that I've seen before include "Jenny's often like this" , "Idiotic" , "Foolish like donkey" , "Daft" , "Senseless" .)