Not soft or yielding to pressure (4)
I believe the answer is:
firm
'not soft or yielding to pressure' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for firm that I've seen before include "Secure - business partnership" , "Unyielding; company" , "securely rooted" , "Company; steady" , "Resistant to pressure" .)