Crossword Genius

Initially, applicant with medical degree on day left a covering letter (6)

Ross

There are a few possibilities for this:

'covering letter' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

'initially applicant with medical degree on day left a' is the wordplay.
'initially' indicates taking the first letters.
'with' is a charade indicator (letters next to each other).
'medical degree' becomes 'MB' (Bachelor of Medicine).
'on' indicates putting letters inside (as in clothing 'on' a person).
'day left' becomes 'oe' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letter of 'applicant' is 'a'.
'mb' going around 'oe' is 'moeb'.
'a'+'moeb'+'a'='AMOEBA'

(Other definitions for amoeba that I've seen before include "cell for life" , "shape-shifter" , "Water protozoan" , "Simplest animal form" , "Microscopic organism consisting of a single cell changing constantly" .)

I've seen this clue in The Guardian.
Want a hint initially instead of a full solution? Install my app