Being inflexible is difficult (5)
I believe the answer is:
stiff
I believe this clue is a double definition.
'being inflexible' is the first definition.
('stiff' can be similar in meaning to 'inflexible')
'difficult' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for stiff that I've seen before include "Unyielding" , "Not easily bent" , "Firm" , "Difficult" , "Starched, rigid" .)