Fat from a sheep's back, nothing on a lamb's head (7)
I believe the answer is:
lanolin
'fat from a sheep's' is the definition.
'lanolin' can be an answer for 'fat' (I've seen this before). I am unsure of the 'from a sheep's' bit.
'back nothing on a lamb's head' is the wordplay.
'back' is a reversal indicator.
'nothing' becomes 'nil' (eg nil is zero in football).
'head' suggests taking the first letters.
The initial letter of 'lambs' is 'l'.
'nil'+'on'+'a'+'l'='nilonal'
'nilonal' in reverse letter order is 'LANOLIN'.
Can you help me to learn more?
(Other definitions for lanolin that I've seen before include "Oil extracted from wool" , "Grease on sheep's wool" , "substance found in ointments" , "something on the Fleece" , "Wool extract used in some ointments" .)