Unhealthily pale friend on top (6)
I believe the answer is:
pallid
'unhealthily pale' is the definition.
(I've seen this before)
'friend on top' is the wordplay.
'friend' becomes 'pal' (pal is a kind of friend).
'on' says to put letters next to each other.
'top' becomes 'lid' (lid is a kind of top).
'pal'+'lid'='PALLID'
(Other definitions for pallid that I've seen before include "Feeble, insipid" , "Colourless, wan" , "Pale complexion because of poor health" , "White in face, perhaps ill" , "Sickly pale" .)