Start up and set out to get fat (4)
I believe the answer is:
suet
'fat' is the definition.
(I know that suet is a type of fat)
'start up and set out' is the wordplay.
'start' means to remove the last letter (I've seen 'start of' mean this).
'and' is a charade indicator (letters next to each other).
'out' indicates an anagram (out can mean wrong or inaccurate).
'up' with its last letter removed is 'u'.
'u' put after 'set' is 'setu'.
'setu' anagrammed gives 'SUET'.
'to get' is the link.
(Other definitions for suet that I've seen before include "Edible animal fat" , "Fatty stuff used in cooking" , "Hard fat from beef and sheep" , "Solid fat used in cooking" , "Hard edible fat" .)