Fool about to get at chap almost drunk (3,3,4)
I believe the answer is:
act the goat
'fool about' is the definition.
(I've seen this before)
'to get at chap almost drunk' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'drunk' is an anagram indicator.
'to'+'get'+'at'+'chap'='togetatchap'
'togetatchap' with its final letter taken off is 'togetatcha'.
'togetatcha' anagrammed gives 'ACT THE GOAT'.
(Other definitions for act the goat that I've seen before include "Behave stupidly" , "clown" , "Fool around" .)