Overindulge in irrational complaint (3,3)
I believe the answer is:
pig out
'overindulge' is the definition.
(synonyms)
'irrational complaint' is the wordplay.
'irrational' becomes 'pi' (an irrational number).
'complaint' becomes 'gout' (gout is a kind of disease).
'pi'+'gout'='PIG OUT'
'in' acts as a link.
(Other definitions for pig out that I've seen before include "Eat too much food" , "To eat a lot may" , "overindulge" , "feast" , "go on a binge" .)