Cheers enveloping half of them in passing (2,3,3)
I believe the answer is:
by the bye
'in passing' is the definition.
(eg mentioning something by the bye or in passing)
'cheers enveloping half of them' is the wordplay.
'cheers' becomes 'bye-bye' ('cheers' can be used to say goodbye).
'enveloping' means one lot of letters goes inside another.
'half of' means to take half the letters.
'them' with half the letters taken is 'th'.
'byebye' enclosing 'th' is 'BY THE BYE'.
(Other definitions for by the bye that I've seen before include "In passing" , "Incidentally" , "Parenthetically" .)