One missing from clear diagram (4)
I believe the answer is:
plan
'diagram' is the definition.
(I've seen this in another clue)
'one missing from clear' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for plan that I've seen before include "Blueprint" , "Scale drawing; scheme" , "Contrive" , "Method laid out for accomplishing something" , "Scheme, design" .)