Small piece not completely in code (5)
I believe the answer is:
morse
'code' is the definition.
(Morse code)
'small piece not completely' is the wordplay.
'small piece' becomes 'morsel' (I've seen this before).
'not completely' means to remove the last letter (the word is incomplete).
'morsel' with its last letter removed is 'MORSE'.
'in' acts as a link.
(Other definitions for morse that I've seen before include "& 24 Across Encrypted language" , "TV inspector" , "10 [SLEUTH]" , "used to transmit messages" , "One who enjoyed solving code" .)