E Musk welcomes pressure (7)
I believe the answer is:
epsilon
'e' is the definition.
(equivalent of E in the Greek alphabet)
'musk welcomes pressure' is the wordplay.
'musk' becomes 'Elon' (Elon Musk).
'welcomes' indicates putting letters inside (in sense of 'invites in').
'pressure' becomes 'psi' (pounds per square inch).
'elon' going around 'psi' is 'EPSILON'.
(Other definitions for epsilon that I've seen before include "Greek letter - pinoles (anag)" , "character in Greece" , "Greek character" , "5th letter of the Greek alphabet" , "Foreign character" .)