Inflexible press (4)
I believe the answer is:
iron
I believe this clue is a double definition.
'inflexible' is the first definition.
(I've seen this before)
'press' is the second definition.
(I know that press can be written as iron)
(Other definitions for iron that I've seen before include "Main part of steel" , "Smooth - driver?" , "Metal obtained from haematite" , "inflexible > club" , "Revolver" .)