Fed gruel initially by staff (1-3)
I believe the answer is:
g-man
'fed' is the definition.
(I've seen this before)
'gruel initially by staff' is the wordplay.
'initially' indicates taking the first letters.
'by' says to put letters next to each other.
'staff' becomes 'man' (as in staffing or manning a particular place).
The initial letter of 'gruel' is 'g'.
'g'+'man'='G-MAN'
(Other definitions for g-man that I've seen before include "FBI agent" , "Officer of the FBI in USA" .)