Friend beginning to conceal unpleasant smell (4)
I believe the answer is:
chum
'friend' is the definition.
(I know that chum is a type of friend)
'beginning to conceal unpleasant smell' is the wordplay.
'beginning to' indicates taking the first letters.
'unpleasant smell' becomes 'hum' (hum can mean a disgusting smell).
The initial letter of 'conceal' is 'c'.
'c'+'hum'='CHUM'
(Other definitions for chum that I've seen before include "Pal, buddy" , "Close friend (colloq.)" , "Salmon" , "Informally, a close friend" , "Good friend" .)