Becomes touchy when one pours the cat a drink (4,2)
I believe the answer is:
acts up
'becomes touchy when one pours' is the definition.
I can't judge whether this defines the answer.
'cat a drink' is the wordplay.
I cannot really see how this works, but
an anagram of 'cat' is 'act' which is present in the answer.
'drink' could be 'sup' (to sup is to drink) and 'sup' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'the' acts as a link.
Can you help me to learn more?
(Other definitions for acts up that I've seen before include "Malfunctions" , "Behaves in a troublesome way" , "Fails to function properly or behaves badly" , "Causes trouble or misbehaves" , "Carries on" .)