Top actors Rickman and Firth getting fat (7)
I believe the answer is:
lanolin
'fat' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lanolin that I've seen before include "Wool-fat" , "Ointment containing wool fat" , "Grease on sheep's wool" , "Oily substance on wool" , "Fat extracted from wool" .)