FA join? That's handy! (2,3)
I believe the answer is:
of use
'handy' is the definition.
(I've seen this in another clue)
'fa join?' is the wordplay.
'fa' becomes 'o' (short for Fanny Adams, meaning nothing or 0).
'join?' becomes 'fuse' (I've seen this in another clue).
'o'+'fuse'='OF USE'
'that's' acts as a link.
(Other definitions for of use that I've seen before include "Helpful" , "Employable" , "Serviceable" , "Advantageous" , "practical" .)