Year employed by group -- great, on the whole (2,3,5)
I believe the answer is:
by and large
'on the whole' is the definition.
(both can mean 'generally')
'year employed by group great' is the wordplay.
'year' becomes 'y'.
'employed by' is an insertion indicator (inserted letters used or employed inside).
'group' becomes 'band' (band can mean a group of people).
'great' becomes 'large' (synonyms).
'y' placed into 'band' is 'byand'.
'byand'+'large'='BY AND LARGE'
(Other definitions for by and large that I've seen before include "Speaking generally" , "In general" , "All things considered" , "On the whole" , "In the main" .)