It's difficult, being inflexible (5)
I believe the answer is:
stiff
I believe this is a triple definition!
'it's difficult' is the first definition.
(I've seen this before)
'being' is the second definition.
(stiff is a kind of being)
'inflexible' is the third definition.
(similar in meaning)
(Other definitions for stiff that I've seen before include "Frigid" , "Inflexible" , "tough" , "Awkward" , "Starched, rigid" .)