Person originally employed in Crosby (5)
I believe the answer is:
being
'person' is the definition.
(I've seen this in other clues)
'originally employed in crosby' is the wordplay.
'originally' suggests taking the first letters.
'in' indicates putting letters inside.
'crosby' becomes 'bing' (singer Bing Crosby).
The initial letter of 'employed' is 'e'.
'e' going within 'bing' is 'BEING'.
(Other definitions for being that I've seen before include "Person" , "substance" , "A living creature" , "A matter of 9 in the play" , "Existing" .)