Hardly fair upsetting a large number, by Jiminy! (3,7)
I believe the answer is:
not cricket
'hardly fair' is the definition.
(something unfair is 'just not cricket')
'upsetting a large number by jiminy' is the wordplay.
'upsetting' says the letters should be written backwards.
'a large number' becomes 'ton'.
'by' means one lot of letters go next to another.
'jiminy' becomes 'cricket' (Pinocchio character Jiminy Cricket).
'ton' backwards is 'not'.
'not'+'cricket'='NOT CRICKET'
(Other definitions for not cricket that I've seen before include "so unfair" , "Unsporting" , "Almost all games are" , "description of all but one type of insect" .)