Constant inclination is a pain (5)
I believe the answer is:
cramp
'pain' is the definition.
(I have seen 'Muscular pain' mean 'cramp' so perhaps 'pain' could also mean 'cramp')
'constant inclination' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'inclination' becomes 'ramp' ().
'c'+'ramp'='CRAMP'
'is a' acts as a link.
(Other definitions for cramp that I've seen before include "Severe muscular pain" , "hamper" , "development of vice" , "Give too little space to" , "Contract" .)