Almost all sale change in farm is an unnecessary warning (5,5)
I believe the answer is:
false alarm
'an unnecessary warning' is the definition.
(I've seen this before)
'almost all sale change in farm' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'change' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'all' with its last letter taken off is 'al'.
'al'+'sale'='alsale'
'alsale' anagrammed gives 'alseal'.
'alseal' placed inside 'farm' is 'FALSE ALARM'.
'is' is the link.
(Other definitions for false alarm that I've seen before include "Panic over" , "unfounded scare?" , "Warning proving unnecessary" .)