Being acid can make one endlessly ratty! (4)
I believe the answer is:
tart
'being acid' is the definition.
(tart can mean sharp or acid in taste)
'make one endlessly ratty' is the wordplay.
'make' indicates anagramming the letters.
'one endlessly' means to remove the last letter.
'ratty' with its final letter taken off is 'ratt'.
'ratt' is an anagram of 'TART'.
'can' acts as a link.
(Other definitions for tart that I've seen before include "Acid-tasting; pastry" , "easy girl" , "The pastry sounds bitter" , "Professional woman" , "Open cake with jam etc" .)