About inclination to be a pain (5)
I believe the answer is:
cramp
'a pain' is the definition.
(I have seen 'Muscular pain' mean 'cramp' so perhaps 'pain' could also mean 'cramp')
'about inclination' is the wordplay.
'about' becomes 'c' (abbreviation for circa).
'inclination' becomes 'ramp' (an inclined surface).
'c'+'ramp'='CRAMP'
'to be' acts as a link.
(Other definitions for cramp that I've seen before include "Painful muscle problem" , "Form of muscular pain" , "common among writers" , "Contract" , "development of vice" .)