Guts are key at the outset (6)
I believe the answer is:
entera
'guts' is the definition.
(I've seen this before)
'are key at the outset' is the wordplay.
'are' becomes 'a' (short for 'are', historical unit of measurement).
'key' becomes 'enter' (key on a computer keyboard).
'at the outset' means one lot of letters go next to another.
'a' after 'enter' is 'ENTERA'.
(Other definitions for entera that I've seen before include "Intestines (Greek), neater (anag.)" , "Digestive tracts; neater (anag.)" , "guts" .)