Fats make protein-building acid non-active (6)
I believe the answer is:
domino
'fats' is the definition.
(I've seen this in another clue)
'make protein-building acid non-active' is the wordplay.
'make' becomes 'do' (do can mean to make or prepare).
'protein-building acid' becomes 'amino' (I've seen this in another clue).
'non' indicates named letters should be taken away.
'active' becomes 'a' (grammatical abbreviation).
'amino' with 'a' taken out is 'mino'.
'do'+'mino'='DOMINO'
(Other definitions for domino that I've seen before include "One of 28 small pieces in a game" , "One falls, all fall" , "Loose cloak; game piece" , "Playing piece; mask" , "one coming down with many others?" .)