Turned irrational after small drink (3)
I believe the answer is:
sip
'drink' is the definition.
(sipping is a kind of drinking)
'turned irrational after small' is the wordplay.
'turned' says the letters should be written backwards.
'irrational' becomes 'pi' (an irrational number).
'after' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pi'+'s'='pis'
'pis' backwards is 'SIP'.
(Other definitions for sip that I've seen before include "Small mouthful of liquid" , "Take in slowly" , "Smallest drink" , "A small taste" , "Drink in tiny mouthfuls" .)