Manager back in harness, so busy (4)
I believe the answer is:
boss
'manager' is the definition.
(I've seen this before)
'back in harness so busy' is the wordplay.
'back' says the letters should be written backwards.
'in' indicates a hidden word.
'ssob' is hidden within 'harness so busy'.
'ssob' in reverse letter order is 'BOSS'.
(Other definitions for boss that I've seen before include "designer" , "One who manages" , "Leader; stud" , "Informal employer or superior" , "Take charge of" .)