It's for broken sink, primarily (5,2,3)
I believe the answer is:
first of all
'primarily' is the definition.
(thesaurus)
'it's for broken sink' is the wordplay.
'broken' indicates anagramming the letters.
'sink' becomes 'fall' (I've seen this in another clue).
'its'+'for'='itsfor'
'itsfor' with letters rearranged gives 'firsto'.
'firsto'+'fall'='FIRST OF ALL'
(Other definitions for first of all that I've seen before include "Before anything else" , "Prime" , "initially" .)