Oily secretion from extremely sore bottom (5)
I believe the answer is:
sebum
'oily secretion' is the definition.
(I've seen this before)
'extremely sore bottom' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'bottom' becomes 'bum' (synonyms).
'sore' with its centre taken out is 'se'.
'se'+'bum'='SEBUM'
'from' is the link.
(Other definitions for sebum that I've seen before include "Oily skin secretion" , "which will cover the skin" , "Oil on skin and hair" , "oily stuff" , "Lubricant" .)