Record in a specially apt key (4)
I believe the answer is:
tape
'record' is the definition.
(I know that record can be written as tape)
'specially apt key' is the wordplay.
'specially' is an anagram indicator (I've seen this in other clues).
'key' becomes 'e' (musical key).
'apt' anagrammed gives 'tap'.
'tap'+'e'='TAPE'
'in a' acts as a link.
(Other definitions for tape that I've seen before include "This is often stretched across the finishing line of race" , "that may be red" , "Breast this first to win foot-race" , "Sticky strip" , "Common medium for recording" .)