Constant irrational hunger (7)
I believe the answer is:
craving
'hunger' is the definition.
(I've seen this before)
'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 "Great desire" , "longing" , "Intense feeling of desire" , "Hunger" , "Strongly desiring" .)