Essential as in pen that's disposable (5)
I believe the answer is:
basic
'essential' is the definition.
(similar in meaning)
'as in pen that's disposable' is the wordplay.
I cannot quite see how this works, but
'as' is within the answer.
'pen' could be 'bic' (pen manufacturer) and 'bic' is present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for basic that I've seen before include "Simple form of language" , "Plain" , "Key" , "Introductory" , "Spartan" .)