Constant irrational desire (7)
I believe the answer is:
craving
'desire' is the definition.
(craving is a kind of desire)
'constant irrational' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'irrational' becomes 'raving'.
'c'+'raving'='CRAVING'
(Other definitions for craving that I've seen before include "Intense feeling of desire" , "longing" , "Powerful desire" , "Great desire" , "Hunger" .)