The first thing in making tarts (5)
I believe the answer is:
start
'the first' is the definition.
('start' can be a synonym of 'first')
'thing in making tarts' is the wordplay.
'tarts' can be anagrammed to 'START'.
Though, I'm not sure how this is indicated.
Can you help me to learn more?
(Other definitions for start that I've seen before include "Outset" , "Commence" , "Begin - jump" , "Begin to recoil?" , "Onset" .)