A busy worker sent in for shirker? (8)
I believe the answer is:
absentee
'for shirker?' is the definition.
(I've seen this before)
'a busy worker sent in' is the wordplay.
'busy worker' becomes 'bee' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'bee' enclosing 'sent' is 'bsentee'.
'a'+'bsentee'='ABSENTEE'
(Other definitions for absentee that I've seen before include "one's failed to show up" , "One away from work, etc" , "are you fonder of him?" , "One who should be but isn't present" , "One is not present when required" .)